Aid climbing is the grand patriarch of all rock climbing techniques. In the ol' days climbing and aid climbing were synonymous. Originally, aid climbers made upward progress by driving pitons into weaknesses in the rock, clipping a ladder-like device called an étrier (or aider) to the piton, standing up on it and doing it again. This is still the primary technique except today aid climbers have more gadgets, and the sport has evolved to include "clean" aid climbing. Clean aid uses removable gear that doesn't need to be hammered in and yanked out and thus does no damage to the rock.
An aid climb usually involves an extended foray on exposed vertical terrain and requires considerable focus and concentration. Aid climbing takes a special sort of mind set. Aid climbers should be patient and tenacious, or perhaps more accurately, should enjoy robotic repetition and possess a mule-like stubbornness.
Using clean aid technique, any route can be climbed, but please note, it is important not to hammer anything, or alter the rock in any way, on a route that can be free climbed (climbed without standing on or pulling on any gear). When learning aid skills, even an accomplished climber should begin on a single-pitch route. This affords a chance to fully understand the laborious process of aid climbing and the unique dynamics involved. From here aid climbers may progress to long multi-pitch routes lasting all day or stretching into many, many days and nights.
In North America aid routes are graded from A0 to A5. Traditionally A0 has meant the absence of holds or features in the rock that would make free climbing possible. However, as climbers continue to improve many longstanding aid routes are now "going free.". Unlike many other route grading systems this is a closed rating. Meaning, A5 is the hardest possible rating and there will never be an A6. How hard is hard? Essentially, an A5 route means the climbers are constantly in a position where a fall has a very good chance of resulting in death for the leader and possibly his partner. Serious stuff.
As you may have guessed, an aid climber's gear selection is enormous. The bulk of the gear consists of a trad climber's rack including sets of nuts and cams. Aid specific gear might include: pitons, hooks, copperheads, ascenders, hauling pulleys, aiders, daisy chains, wall hammers, haul bags, and possibly a portaledge. An aid climbing team carries at two or more ropes: a dynamic rope for the lead climber, and other static and dynamic ropes for hauling, rapping, and possibly fixing in place. It all adds up to a potentially huge mental, physical, and financial commitment!