In informal settings, Brigadier Generals are typically referred to simply as "General".
A Brigadier General may be also referred to as a "one-star General" to distinguish it from the other ranks of General; so named for the one star rank insignia worn on the uniform.
Given that other officer ranks command the same unit groupings as their modern day U.S. counterparts do, it is highly probable that a Brigadier General commands brigade-sized units. This has yet to be observed in the Halo universe. A Brigadier General may also serve as a staff officer for higher commands.