The Timer sends out a pulse at a user-specified time interval (right click to change). The Timer sends out a redstone pulse to its front, left and right. It can be stopped by powering it from its back, left, or right.
When the Timer recieves a negative (on-to-off) edge to its back, left, or right, the timer will reset to the beginning of its time instead of resuming at the point where it left off. This can be handy because you can send a quick pulse to a timer to cause it to go back to the beginning without triggering an "on" state on any of its outputs.
A good use for it is in a nuclear reactor safety device.
If your reactor is a -SUC and is being suplied with ice from pneumatic tubes, put an item detector somewhere along the line of pneumatic tubes. Place a redstone signal near it (preferably have your ice coming through one a stack at a time) so that every time a stack of ice comes through, it emits a redstone signal.
On another part, have a timer connected to a RS latch and set the timer to slightly longer than the duration in which your reactor needs ice. In this way have the RS latch output to a reactor shutoff.
The system I have implemented will shut off the reactor if the item detector does not receive a stack of ice every 1.5 seconds. The reactor will stay off until a button is pressed.
A timer is also good for use with any automatic farms that involve pistons; this way you do not have to press a button. The plants can then be pushed into Obsidian Pipes for collecting or whatever use they are needed for.