1) Boost - They increase your overall signal as well as adding some degree of compression. This gives your notes a more consistent attack that's like a hot knife through butter, even with a clean signal.
2) EQ - Unless you have a multi-band preamp (like those found on acoustic-electric guitars), you more likely find that preamps just boost a single band. For example, EMG's SPC ("Strat Presence Control") gives a boost around 5 kHz. Other models modify other bands similarly.
The marketplace is incredibly varied, so there are more and wilder things than the few I have featured here in most of these categories. Look around the web if you are interested in some particularly experimental devices.