When you go in for a yearly exam they do a few things. First they will to a pelvic exam to make sure everything feels as it should and nothing is inflamed. This is also an important thing to have done. They can find and diagnose many issues simply by doing this. And is a good place to start.
With the pap, they use what looks like a long qtip to take a sample of the cells on your cervix. (Some women find it uncomfortable, I've never had a problem with it). They then send the sample to a lab who checks for 'abnormal' cells. This is how they test for many std's.
The whole visit lasts about 10-15 minutes and is completely worth the time. Planned parenthood does them for free, if you don't have insurance. This is also the time they normally talk to you about birth control and discuss wether your current method is working and making sure you don't have any questions. (mine normally goes over Plan B with me, just to make sure I'm aware and comfortable with it).