What is a bot?
The Viewer you use to access the Second Life® world (and the fact that the Viewer code is open source) allows for the creation of custom Viewers that can log in without a human operator and perform the same actions as any normal avatar inworld. Avatars controlled by these custom, automated Viewers are called "bots" (a contraction of "robot").
Bots can be hard to spot; sometimes they live very high up in the air, or are so fast that they have gone by before you're aware of them. If you find yourself somewhere that seems to be full of people, and yet nobody is talking or moving about, it is likely that some or all of the avatars you see are bots. Don't be alarmed if they don't respond to your chat!