If you've used an RC system it's easy: timers pretend to be receivers.

At their most basic, they provide a throttle signal to the ESC that starts low, goes high for your desired flight time, then goes low again.

In the case of KR timers, they also monitor the motor speed and regulate it.

In the case of Igor Burger's timers, they can also monitor the flight speed of the plane (with an accelerometer) and adjust the motor speed.

Some of them can also do landing gear.

I'm going to stop now.  It's either that, or write a book.  I hope this helps.

The cookbooks has a timer by Will Hubin. What are those? How do they work?

Timers in general: The way I understand it, ESC's can perform similar functions such as shut off or RPM regimes, right? What are the differences between performing the function in esc or the timer.

Look at this:

http://stunthanger.com/smf/index.php?topic=17035.0

The Will Hubin different timers can be purchased from him directly. Email him at whubin@kent.edu and describe what ESC you plan to use and he can advise you about a timer. His run from about ten to twenty dollars except that his FM-9 series timers need a programmer at $70-80.

Keith Renecle's timers can be purchased from Eric Rule at RSM Distribution. rsm1rule@rsmdistribution.com They can be used with an inexpensive ESC and still have governing of the RPM. His timers also need a programmer. I bought one recently with a programmer for $52.50

In both cases you only need to buy the programmer only once and then the additional timers can use the same programmer.

I don't know if Igor is marketing his timers or not.

These to me are the most popular timers right now.

Just ask Will to email a copy of his timer and programmer brochure, the prices are there. I like the FM-9 timer with remote start button. I have a FM-9V (+/ 500 rpm remote adjustment)on a setup but don't find that I use the remote rheostat rpm adjuster. The programmer box is easy to connect to the timer and make the changes if I need more rpms. I don't find it necessary to make a rpm change once the motor dialed in to fly the plane.
Bob