Unexpectedly, the issue of the use and misuse of antibiotics came up today. This annoyed me, but rather than yell at people, I'll put up a little primer here. I guess spending my time with biologists, chemists, biochemists, microbiologists, and toxicologists means I forgot most people don't spend much time thinking about these things.
Briefly, antibiotics are chemicals that kill bacteria; there are a few that will kill fungi, protists, or other organisms, but the vast majority of antibiotics prescribed for treatment of human illness will kill bacteria. Mode of action varies; many antibiotics interfere with bacterial cell division, which is a process different enough from how animal cells reproduce (e.g. cell wall synthesis - we don't have cell walls, some bacteria do) to hurt the bacteria without hurting the host (i.e. you). The net result of a course of antibiotics is to kill a very large fraction of the bacteria responsible for an infection, or to prevent them from reproducing long enough for the immune system to get ahead of the bacteria and wipe them out.
Antibiotics are an obvious agent of selection. Any bacterial cell that can avoid or reduce the effects of the antibiotic - for example, by producing an enzyme that destroys the antibiotic before it can interfere with the bacterial cell - will have a clear advantage, and its offspring will carry that resistance. This is why it is VERY IMPORTANT to take the FULL COURSE of a round of therapeutic antibiotics. The symptoms of the infection - pain, puss, whatever - might clear up in a few days, but the antibiotics will be in sufficient number of pills for more than a week. KEEP TAKING THE ANTIBIOTICS. The bacteria are down but not out after those first few days; most of the surviving cells may be resistant. In many cases, resistance means reduced but not eliminated effect of the antibiotics, so those nasty cells are still getting hurt, just not outright killed as their less fortunate compatriots were. Keep hammering them, and the immune system will clean up the mess - few bacteria can withstand the double-blow of compromised reproductive ability and roving bands of killer white blood cells.
Here's a picture of a neutrophil, a type of white blood cell (immune system) that targets bacteria and fungi (photocredit to Bob Blaylock and this Wikipedia page).