It all depends on what you're fishing for.
It's hard to define actions as it actually varies quite a bit. One companies medium heavy is another companies heavy. It also varies between different lines in each company.
Also light, medium... are not actions. Those are powers. Action is fast, extra fast, moderate and so on.
Best advice I can give you is look at what types of fish you're fishing for, whether you're fishing from the bank or a boat, how tall you are and so on. Then pay attention of the ratings of the rod. Line weight, lure weight and so on.
I prefer a moderate to fast action rod myself. Also since I'm fairly tall and fish from a CC boat I really prefer longer rods.
Actions break down like this for the most part.
Extra fast, the top 1/4 of the rod bends the most.
Fast, the top 1/3 of the rod bends the most.
Moderate, the top 1/2 of the rod bends the most, also called classic action as it most closely duplicates a fiberglass rod.
Slow, the rod bends evenly all the way to the handle.
When I moved down here I realized that my salmon/steelhead rods weren't going to cut it. I now fish TFO rods in the 7' to 7'6" range. I've got powers from medium light to heavy.
This Veteran heavily medicated for your safety. Keep me fishing and all will be well.