Timers

System.Threading.Timer

Added in .NET 1.1
Lets you contonuously call a delegate at specified time intervals.
Lets you schedule a single cell to a delegate.
The delegate is executed on a ThreadPool thread

System.Timers.Timer

Added in .NET 1.1
This is just a wrapper for System.Threading.Timer with some extra features
This raises an event on a ThreadPool thread by default.
This has some additional features
An interval property instead of a change method
An elapsed event instead of a callback delegate
An enabled property to start and stop the timer
Start and Stop methods
An AutoReset flag to indicate a recurring event

System.WIndows.Forms.Timer