In the first three series of Andromeda, the Magog were hairy creatures, much like walking rats. In the fourth series, they change to looking more like prehistoric man. Magog capture their prey and infest them with their larvae much like the Wirrn in Doctor Who and the creatures from alien. Although not explicitly described, it is a good bet they eat their young hosts, such as humans.
They inhabit a WorldShip, an interconnecting collections of planets with a sun at the centre. Their attack ships are capable of digging into their preys spaceships to get what they want. Rev Bam, a Magog is a member of Andromeda Ascendants crew for the first and part of the second series. All the Magog are of one gender, there doesn't seem to be any female magogs. Leading the Magog is a Spiritual being more energy than human known as the Abyss.