Accounts used by approved bots to make pre-approved edits can be flagged as such. Bot accounts are automated or semi-automated, the nature of their edits is well defined, and they will be quickly blocked if their actions vary from their given tasks, so they need less scrutiny than human edits.
For this reason, contributions from accounts with the bot flag ('bot' user group) are not displayed in recent changes or watchlists to users who have opted to hide bot edits. Minor edits made by bot accounts to user talk pages do not trigger the "you have new messages" banner. Bot accounts can query the API in batches of 5,000 rather than 500.
See Special:ListUsers/bot for a list of the 409 bots.