There are 2 kinds of pressure cookers. The manual one and the electronic one.
The manual one cooks using a stove top. You must be careful to watch that the pressure does not rise beyond the manufacturers safety limits, otherwise it may explode. The electonic of electric ones, plug in and you can simply set the timer and walk away. Mine has setting for Beef, chicken, soup, fish, etc. each one will go for a certain length of time and then automatically shut off.
There is a rocker on the top of each kind of cooker, and if the pressure builds up too much, it can be quick released by using the rocker to make the steam escape. When the food has finished cooking, this rocker can be used for a "quick release" or you can just let the cooker sit for half an hour or so to release on it's own.
This is a very simple explanation. Just google pressure cookers and see what comes up......
I love my electronic one because I don't have to stand in the kitchen to make sure it does not over pressurize.
Pressure cooking and slow cooking are the two easiest ways to cook meals. The slow cooker, you just put everything in and go to work, when you come home, dinner is all ready.
There is a bit more to pressure cooking, but if you have a tendancy to forget to take meat out of the freezer, of it you like to make compound meals (sich as beans, or soups and stews etc (great at this time of year) then there is nothing better.
I once made a bean soup where the various beans and pea all cooked for different lengths of time and it was a real pain. I had to keep letting the cooker sit so the pressure would go down so I could open it to put the next lot in. Needless to say it did not save any time at all!!!!
So now everything that I put has to cook around the same time. This means some harder veggies may have to be cut a bit smaller, so it is all cooked evenly. Hope this helps.