In general, match the power of the rod to the size fish you are targeting, and the "action" or flex of the rod to the type of baits that you use and how you fish them.
One manufacturer's M can be anothers ML or MH, so it is difficult to answer such a broad question. If you have specific rod brands or models in mind, maybe asking which rod in those lines people recommend could help narrow your search.
Good luck.
Dan