When .NET first came out, the framework only had abstractions for what seemed like a handful of Windows features. Developers were required to write their own abstractions around the Windows features that did not have abstractions. Working with these features required you to work with unmanaged resources in many instances. Unmanaged resources, as the name suggests, are not managed in any way by the .NET Framework. If you don't free those unmanaged resources when you're done with them, they'll leak. Unmanaged resources need attention and they need it differently from managed resources. Managed resources, by definition, are managed by the .NET Framework and their resources will be freed automatically a great proportion of the time when they're no longer in use. The Garbage Collector (GC) knows (or is "told") what objects are in use and what objects are not in use.