The Pomodoro Technique is a productivity technique where you alternate between 25 minutes of work and 5 minutes of break time.

It gets its name from a kitchen timer shaped like a tomato (pomodoro in Italian).

The basic intuition for the pomodoro technique is that:

People concentrate most effectively shortly after a break
Most people are not innately good at noticing when they are not concentrating effectively

By setting an actual, physical timer (for work time as well as for breaks), people are more likely to work effectively. Traditionally, this is set for 25 minutes of work and 5 minutes of time, though for some tasks other lengths may be appropriate. Experimentation is encouraged.

The 'full' pomodoro technique consists of 6 steps (from wikipedia):

Decide on the task to be done.
Set the pomodoro timer (traditionally to 25 minutes).^[1]
Work on the task.
End work when the timer rings and put a checkmark on a piece of paper.^[5]
If you have fewer than four checkmarks, take a short break (3–5 minutes) and then return to step 2; otherwise continue to step 6.
After four pomodoros, take a longer break (15–30 minutes), reset your checkmark count to zero, then go to step 1.

Some who use the technique also encourage focussing on a single task for each pomodoro, or reviewing the work of the previous pomodoro for the first few minutes of each (a type of spaced repetition).

Several tools are available to manage pomodoro timers. Pomofocus is a good, simple, online option. Flow (Mac/iPhone/iPad) and Pomodoro Flow (windows) are some examples of downloadable ones.