Named licenses are setup so that each user would have there own dedicated license. They are guarenteed access to the system because they don't share the license with anyone else. You can only have as many users as you have licenses. These are the least expensive license option, but also the most inflexible.
Concurrent licenses allow for license sharing among your users. If you have 5 concurrent access licenses, you can only have 5 users accessing the system at any given time. The advantage here is that you can have as many users as you need, but not have to invest in alot of Named licenses for those casual users who may only log in once a month. These are more expensive then Named, but more flexible.
You can mix Named and Concurrent in a deployment. This is nice because it allows for frequent users to have guarenteed access, yet you can have concurrent access for casual users.
~Brian