You set the time intervals at which you want to take breaks and how long you want them to last (by default they're set to 10 minutes). During the breaks you can't work on the computer unless you enter a password, and depending on the level of security you set, it's possible to not even give yourself the option of entering a password to make sure you take the entire break.
Other setup options let you disconnect the Internet during the break (just in case), make a sound before going to the break screen, change the break screen to leave it black or clear, and much more.
UsefulRest is a very peculiar app that may be very useful for people who don't have the force of will to get up from the computer by themselves.
30-day trial period.