Bots (robots) are go playing computer programs. They are registered as normal users on DGS and are generally capable of playing with anyone interested, but they may have particular game parameters programmed in them, such as a particular board size.
You can detect currently active bots by their robot type icon . They may also have user-ids that contain the word (ro)bot or the name of some well known go-playing program like gnugo, fuego, mogo, etc.
The bots on DGS start games if invited according to their game preferences via the DGS invitation system. Depending on their operator, bots may run 24/7 or just at special times of the day or week.
Bots generate their moves without human intervention, though they may need some help during scoring. The strength of a bot depends on its algorithm, its thinking time and the computer or computer cluster on which it runs.