A slow-action rod bends over almost its entire length, well down toward the handle. Among spinning and casting rods, fast actions are most useful when jigging or when fishing plastic worms because the stiffer midsection and butt allow fast, powerful hooksets. Moderate actions will cushion hard strikes and work best with reaction-style lures like crankbaits or spinnerbaits that are fished with a steady retrieve. Slow-action rods are great for casting live baits because their slow, lobbing casts won't snap bait off your hook. This defines the literal strength of the rod.
Power ranges from ultralight through medium on up to ultraheavy.
One rod is substantially stronger than the other, but the basic casting action is the same. This simply means the overall rod weight in ounces. Generally, lighter weight gives better sensitivity in fishing, meaning your ability to feel subtle taps at your bait or lure. And be forewarned that lighter weights often mean high-tech rods at premium prices. To put these three concepts together and actually choose a rod, you'll also need to define as carefully as possible the kind of fishing you want to do.