Record Types are hard to get your head around but are so super powerful that you need to understand them well, and use them - everywhere.
Record Types go hand in hand with Rules for Page Layouts, Profiles, Opportunity stages.
Record Types are only available in Enterprise and up. As with Workflows, and the API it is the main reason to use Enterprise.
Record Types Help document https://help.salesforce.com/apex/HTViewHelpDoc?id=customize_recordtype.htm&language=en
Copy the existing page layout and modify it to suit the record type it will be used for.
We are choosing which picklist values are NOT going to be included for this Record Type. This is usually appropriate for different Lead or Opportunity stages that may be different for different Sales Processes that you do - eg selling widgets has different profiles than selling services.
Now come back to the Record Types menu and click on the Page Layout Assignment button.
This screen is difficult to understand, but once you do it becomes quite easy. It is harder to understand with more record types.
This is where the naming of the page layouts is crucial. If your page layouts are named Clients - Sales and Clients - Admin, then it is easy to set the page layouts to the right record type (clients) and the right profiles (sales or admin).
It's a 3 way relationship
All of these combined, work out which page layout will be displayed to the user.
Note about setting the default so not having the modify the PLA.
Now we are going to put the record type field on the page layouts. Remember to add it to each page layout in exactly the same position.
Your record types are now set up.
The next step is to extend the Record Types with workflows.