Circolog uses a sql-inspired query language. If you know SQL, then you can use "where clauses" in circolog. If you don't know SQL, don't worry: the language is easy enough for you to learn the most basic queries without worrying too much.
You can only filter the rows, you can't sort them or group them in any way.
msg: the string with the main information
prog: also known as "program" sometimes
facility: an integer describing auth, daemon, user, etc.
host: the hostname where the entry originated
time: date in format
sev: an integer describing severity