When you're cooking meats, if you don't cook them thoroughly enough you run the risk of making yourself and anyone else you're feeding sick. Recommended temperatures for different meats are 145°F for whole meats, 160°F for ground meats and 165°F for all poultry.
There are a bunch of different thermometers you can use, but I prefer the old fashioned kind.
Their accuracy can be affected, though, if they're dropped, or if they're not used for long periods of time. That's easily corrected, though. Test their accuracy by immersing them in boiling water. They should read 212°F, like the thermometer in the picture above. (Unless you're at a high altitude, and in that case the boiling point will be lower.) If they don't read 210°F, they can be easily adjusted by turning the screw under the dial.
Also, when checking the temperature of your cooking meat, make sure that you insert the thermometer at least 2/3 of the length of the stem into the meat. That means if you're cooking chicken breasts or burgers, you need to insert the thermometer in the side of the piece. And you're cooking a roast or a whole bird, make sure the thermometer isn't touching the bone, which will be warmer than the rest of the bird.
Happy and safe cooking!