Multi-beam and single beam descriptions can be applied to two different types of systems, Hydrographic depth sounding sonars and side scan sonars.
With Hydrographic depth sounding systems a single beam sonar “looks” straight down and collects a single depth sounding per ping. A Hydrographic multi beam collects numerous individual depths also known as soundings across a wide swath of the bottom with every ping. The hydrographic multi-beam allows the surveyor to get depths of every section of the sea floor. A single beam only collects depths from directly under the boat.
With Side scan sonars, a multi beam side scan allows the user to travel much faster over the bottom without leaving gaps between pings.
There are also sonars which do both called Interferometric or Multi Phase Ecosounder.