In .NET, structures are used to create types with value type semantics. Value types are for example the built in structs like int, long, bool, single and double.
If you are in doubt, don't have any performance problems, or if you just don't know that much about structs, use a class.
A struct should be small (recommended max size is 16 bytes), immutable (doesn't change once it's created), and don't contain references (unless perhaps to strings, which are also immutable).