Has anyone used a DNA Registry?

I've looked at a couple of DNA registries and I must say I'm thoroughly confused. It seems to me that if we shared the same father, it would be easy. However, we are looking for our half brother from our mother. Apparently, maternal DNA can only show lineage, but can't match two siblings from the same mother. I don't get it....I'm not a science whiz. Does anyone on here understand how DNA works and if its even worth pursuing?