With a concurrent license, you are allowed a maximum number of "concurrent users." A user is considered "active" if he/she has used the system in the past hour and has not explicitly logged out. Users with any Collaborator page open in a browser will remain logged in regardless of whether they are actively navigating through the site. Concurrent licensing is more appropriate when you have many users that will use the system only occasionally. In both cases, a "user" is a human being, not tied to a particular machine or client. This means that if someone uses both the Eclipse client, stand-alone client, and command-line client, plus uses Collaborator on a work machine and laptop, that all counts as one "seat" whether fixed or floating.
With a named license, you are allowed a fixed number of "named users." A "named user" is a human being (not a machine) that is active in the past 30 days. Therefore, if some users never log in, or if a user leaves the group and doesn't log in again, that user does not count towards the named license. Named licensing is more appropriate when most users will be using the product daily.
How many licenses do I need?
It's hard to give guidance because it varies quite a bit. Generally the best advice is: Just try Collaborator. There is a report on the User Administration page that tells you exactly how many named and concurrent licenses you would be using right now. Use that empirical information to determine which is best for your team. Typically, a trial doesn't involve everyone, and typically the usage pattern isn't exactly the same as when it will be deployed, so you'll have to estimate. Still, you'll be doing so with some real numbers.
Here are some additional pointers: