EDIT: This isn't quite true for C#, a tag I missed when I answered this question - if there is a more C# specific answer, please vote for that instead!
They all represent integer numbers of varying sizes.
However, there's a very very tiny difference.
int16, int32 and int64 all have a fixed size.
The size of an int depends on the architecture you are compiling for - the C spec only defines an int as larger or equal to a short though in practice it's the width of the processor you're targeting, which is probably 32bit but you should know that it might not be.