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

This is an invisible stopwatch that gives you access to the system clock.
It can be used to count down from a preset time to cause a delay in a program or to repeat an action at prescribed intervals.
Although timer objects ate not visible at runtime, each timer is associated with an event procedure that runs every time the timer's preset interval has elapsed.