The .NET collector has two main modes of operation: concurrent and synchronous (sometimes known as workstation and server). Concurrent garbage collection is used in desktop applications and synchronous is used in server applications such as ASP.NET by default.
In concurrent mode, .NET will try to avoid stopping the running program while a collection is in progress. This means that the total amount that the application can get done in a given period of time is less but the application won’t pause. It’s good for interactive applications where it’s important to give the impression to the user that the application is responding immediately.
In synchronous mode, .NET will suspend the running application while the garbage collector is running. This is actually more efficient overall than concurrent mode - garbage collection takes the same amount of time, but it doesn’t have to contend with the program continuing to run - but means that there can be noticeable pauses when a full collection has to be done.
http://www.simple-talk.com/dotnet/.net-framework/understanding-garbage-collection-in-.net/