Yeast. You should have yeast settling at the bottom of your brews! That's how the beer gets carbonated in the bottles - you (hopefully) added your priming sugar before bottling, which reactivates the yeast to produce more CO2 and a bit more alcohol. Since the bottle cap is a tight seal, the CO2 stays in the beer and gives it all those nice little bubbles.
When you pour your beer into a glass, leave a little bit at the bottom, so the yeast doesn't get in your glass. Tell your friends, too, otherwise they'll think your beer tastes weird.
Some commercial breweries bottle condition, too, which is how people harvest yeast that's often unavailable to purchase (Pacman, e.g., or Bell's house strain).