Power:
Generally used to describe a rod or blank’s stiffness or resistance to bending. Also known as “power value” or “rod weight.” Rods may be classified as Ultra-Light, Light, Medium-Light, Medium, Medium-Heavy, Heavy, Ultra-Heavy, or other similar combinations. Power is often an indicator of what types of fishing, species of fish, or size of fish a particular pole may be best used for. Ultra-light rods are suitable for catching small bait fish and also panfish, or situations where rod responsiveness is critical. Ultra-Heavy rods are used in deep sea fishing, surf fishing, or for heavy fish by weight. While manufacturers use various designations for a rod’s power, there is no fixed standard, hence application of a particular power tag by a manufacturer is somewhat subjective. Any fish can theoretically be caught with any rod, of course, but catching panfish on a heavy rod offers no sport whatsoever, and successfully landing a large fish on an ultralight rod requires supreme rod handling skills at best, and more frequently ends in broken tackle and a lost fish. Rods are best suited to the type of fishing they are intended for.

Flex-Chart

Action:
Where most of the initial flex in a rod blank takes place. Fast Action rods will flex mostly in the upper 1/3rd of their length. Moderate Action rods in the upper 1/2 of their length. Slow Action rods flex along their entire length.
“Action” refers to the responsiveness of the rod to bending force (bending curve), and the speed with which the rod returns to its neutral position. An action may be slow, medium, fast, or a combination (e.g. medium-fast.) Fast Action rods flex most in the tip section. Slow rods flex more towards the butt of the rod.

The construction material and construction method of a rod affects its action. Action, however, is also often a subjective description of a manufacturer; some manufacturers list the power value of the rod as its action. A “medium” action bamboo rod may have a faster action than a “fast” fibreglass rod. Action is also subjectively used by anglers, as an angler might compare a given rod as “faster” or “slower” than a different rod.

A rod’s action and power may change when line weight is greater or lesser than the rod’s specified range. When the line weight used greatly exceeds a rod’s specifications a rod may break before the line parts. When the line weight is significantly less than the rod’s recommended range the line may part prematurely, as the rod cannot fully flex to accommodate the pull of a given weight fish. In fly rods, exceeding weight ratings may warp the blank or have casting difficulties when rods are improperly loaded.

The action refers to how much a rod bends when a fisherman is casting or have a fish at the end of the line. An extra fast action rod bends just at the tip. A fast action bends in the last quarter of the rod. A moderate-fast action rod bends over the last third. A moderate action rod bends over the last half. A slow action rod bends all the way into the handle. Fast action rods allow the fisherman to make longer casts.

Source:  www.rodbuilding.org