I recently had the opportunity to learn a lot more about licencing that I ever wanted to know. So here are some notes about that. This is not a FULL overview of licencing by any stretch of the imagination. Just some points. 

Overview

This is a handy starting point but beware, it is a little out of date. https://docs.google.com/spreadsheets/d/1REUjHOskL7VPFfMqd8ujWkoWFrkmC9Dtqm0viQOb4xc/edit#gid=0 - no Employee community and Portals are not marked as deprecated. 

Salesforce Agreement

For your reading pleasure - the Master Services Agreement https://www.salesforce.com/assets/pdf/misc/salesforce_MSA.pdf

ALL of the agreements http://www.salesforce.com/company/legal/agreements.jsp

AppExchange Apps

Just beware of where apps are accessing your data, transmitting your data to and storing your data. 

From https://appexchange.salesforce.com/userAgreement my emphasis.

That may result in the disclosure, modification or deletion of your data by those providers or their applications. Further, the AppExchange application provider and its agents and partners may collect and use, data pertaining to your configuration and use of the application. Salesforce.com is not responsible for any transmission, collection, disclosure, modification, use or deletion of your data, as described in this paragraph, by or through third-party AppExchange applications or their providers.

Your use of AppExchange applications will be governed by terms you must click-accept during the test drive or installation process. Please read those terms carefully before accepting them. If the AppExchange application is provided by us, you may contact us with any questions about those terms. If the AppExchange application is provided by another company, then you should contact that company directly with any questions about those terms.

OEM Licencing

An OEM application is an application built on top of the force.com platform. It allows companies to build a great application without having to build all the infrastructure first - it is using force.com as the base platform that handles security, user management, creating and displaying data etc. Fundamentally there is very little difference between the force.com platform and Salesforce products like Service Cloud and Sales Cloud. It is more around how the products are licenced, and what is included in the bundle customers pay for. 

The force.com platform has standard objects - Accounts and Contacts, Tasks, Activities, Chatter, Files and Documents, as most software applications have something to do with tracking the details of people and connecting with people. OEM developers build on top of this platform by using force.com Custom Objects. 

OEM applications allow application developers to bundle up their customisations and sell them together with the force.com platform licencing.

OEM Licencing

The main thing about an OEM application is the licencing. See OEM Embeded licences on the Salesforce help site. The OEM Licence, although it gives you the ability to see Leads, Opportunities, Contracts and Cases expressly prohibits the use of these objects. The OEM Embedded licence is "A full Force.com license with contractual restrictions. It prohibits Create, Read, Update, and Delete on Leads, Opportunities, Cases, Solutions, and Campaigns" and "End users can’t develop applications or extend applications by creating custom objects, but they can have access to additional applications as long as those applications are sold with an embedded license". 

The licencing is controlled by the SFDC Service Agreement that is sold with the OEM package. A quick google search shows some of them https://www.google.com.au/search?q=%22SFDC+Service+Agreement%22. Let's have a look in detail at the Financial Force agreement. (I chose Financial Force because it is a Salesforce OEM app). 

SFDC Service Agreement Excerpt. Clause 1 (a)

http://www.financialforce.com/company/cloud-erp/legal/platformtermsofuse-other/ my emphasis. 

Each User subscription to the Reseller Application shall entitle one User to use the Service via the Reseller Application, subject to the terms of this SFDC Service Agreement, together with any other terms required by Reseller.  User subscriptions cannot be shared or used by more than one User (but may be reassigned from time to time to new Users who are replacing former Users who have terminated employment with You or otherwise changed job status or function and no longer require use of the Service).  For clarity, Your subscription to use the Platform hereunder does not include a subscription to use the SFDC Service or to use it in connection with applications other than the Reseller Application.  If You wish to use the SFDC Service or any of its functionalities or services, to use another application other than the Reseller Application, or to create or use additional custom objects beyond those which appear in the Reseller Application in the form that it has been provided to You by Your Reseller, visit www.salesforce.com to contract directly with SFDC for such services. In the event Your access to the Reseller Application provides You with access to the SFDC Service generally or access to any SFDC Service functionality within it that is in excess to the functionality described in the Reseller Application’s user guide, and You have not separately subscribed under a written contract with SFDC for such access, then You agree to not access and use such functionality, and You agree that Your use of such functionality, Your use of applications other than the Reseller Application, or Your creation or use of additional custom objects in the Reseller Application beyond that which appears in the Reseller Application in the form that it has been provided to You by your Reseller, would be a material breach of this Agreement.

This means the following:

• Even though you access to these objects in the Setup, because there is at least one administrator for the OEM app, the use of the Standard Salesforce objects like Opportunities, Leads, Cases, Contracts in Salesforce is not allowed. 
• The creation of the custom objects is not allowed. 
• Even if the reseller created custom objects, if those custom objects are for functionality that is not listed in the reseller's user guide, then it is up to you, as the end user, to not use them. 
• All of this can all be solved by contracting directly with Salesforce for additional licencing on top of your OEM licencing. 

You can buy a simple Salesforce licence that gives you access to a limited amount of Custom Objects (but not CRM Objects), or a Salesforce.com Sales Cloud or Service Cloud licences. 

Custom Objects

It is all about the Custom Objects. You can't create your own Custom Objects without additional licencing, but it looks like you can install as many third party apps as you want. The custom objects contained in a managed package publicly posted on the AppExchange don't count against the limits for your Salesforce Edition. So, just beware of Unmanaged Packages and packages that are listed as Private (eg this as an example) - it says what they are on the details tab of the AppExchange listing.

The Chatter Plus licence gives 10 custom objects but the Salesforce App Bundle gives 200 custom objects for AU$110/user/month. If you are wanting to create a few custom objects, or install unmanaged packages then going with the App Bundle means that you don't have to obsessively worry over exactly how many Objects each user may have access to. Note however, that the Chatter Plus or the App Bundle does NOT entitle users to use Standard "CRM" Objects like Opportunities, Leads, Cases, Contracts etc. You need a full Salesforce.com licence for that. 

Platform Licencing

Similar to OEM licencing, it is all about the custom objects. The use of the Standard Salesforce objects like Opportunities, Leads, Cases, Contracts in Salesforce is not allowed

https://developer.salesforce.com/docs/atlas.en-us.packagingGuide.meta/packagingGuide/isvforce_user_license_comparison.htm

http://www.salesforce.com/au/crm/editions-pricing-platform.jsp

This is great if you are not a Sales-focused company and want to develop your own in-house apps in Salesforce. 

But remember, as Steven Herod says, "if it looks like a duck and quacks like a duck it's a duck". So don't try to re-create Opportunities just to save money. 

You can still install AppExchange Apps. 

I look at this type of app as Dicing up the Salesforce platform - you can have all the features of Salesforce except the chunk of standard objects. (in comparison, I think of Communities licences as Slicing the Salesforce platform - you can have access to all the standard and custom objects you want to give your customers and partners, but only limited records). 

Communities

I won't delve into communities licencing just yet. 

Here is a great post to explain some of the high level details. Salesforce Standard Object Access by Community or Portal license type - LimitException

And the Salesforce comparison of Communities licencing https://help.salesforce.com/apex/HTViewHelpDoc?id=users_license_types_communities.htm&language=en_US