User Agent

The identifier a client sends with every request, and the robots.txt line that names which bot a rule applies to.

Group rules per agent: one block for Googlebot, one for a specific crawler you want to slow down, and a wildcard for everything else.

A bot only obeys the most specific group that matches it, so a wildcard rule is ignored entirely once a named group exists for that agent.

It is how you give different crawlers different access without changing the site itself.

Serving different content based on user agent crosses into cloaking, so the line is worth understanding precisely.