http://www.stcroixrods.com/content.asp?id=70 will explain power vs action to you.
Generally speaking, selection of power will be dictated by the size of lures you're throwing and the type of cover you're fishing (ie thick lily pads/stumps vs sand bottom/mud) and the size of fish you're expecting.
Action is typically governed by the type of lure you're throwing and personal preference. For jigs, plastics, topwaters, and other baits not considered "moving baits" (baits like crankbaits which are constantly on the move), a fast action rod is by far the most popular. In many instances, I like to go even with an extra fast but if you need an all purpose rod, go with a fast.
Moving baits are a slightly different story as it's customary to get a slower action for these to avoid pulling out the hooks during the fight. Most guys will prefer a moderate action if they've got a dedicated crankbait (and some like this for spinnerbaits as well) rod.
Assuming you're fishing in Ontario for bass, if you can only have one rod, I'd go with a fast action medium or medium-heavy powered rod. Keep in mind that different manufacturers will often rate their powers differently (St.Croix ML = Shimano M = G.Loomis 2 power = etc etc). When in doubt, look at lure/line ratings instead of the power rating.