I've used a 35% glycolic peel ordered twice, 7 days apart. The first time I barely applied it and felt nothing during the process, leaving it on my face and chest for 3 min. I didn't notice any difference other than a short-lived redness immediately afterward. (My chest was redder and during the following week some blemishes appeared which is unusual for me there.)
During the 2nd peel, however, I felt stinging of the skin almost immediately. It was certainly tolerable, but I was glad to wash it off 4 minutes in. More redness, a few red marks on my neck, and some places on the face that were very red long after the other skin settled down again. I ended up having some mild peeling in those very red places that took a full 9 days, but the skin underneath is smooth, fresh and freckle-free. I decided because of these irritated places to give my face a 2 week rest, and I will soon give myself another peel now that all is settled again.
My chest area is suffering from a breakout, however, which has me pretty confused. I don't usually have blemishes there, but the peel seems to have brought everything to the surface. Maybe the pores there are getting exfoliated? Not sure what's going on there, so I'm not going to retin-a this area for a bit to prevent further irritation.
Others have given you some good info on what peels do, and the differences, so I won't go there. I have to say 2 weeks into it and I am very happy with the results. My skin looks lighter and smoother, and although I can't attribute it to the peel completely, I believe it has sped up the exfoliation process considerably. It's an easy procedure to do, and inexpensive too. I have read in many places if you have a dark complexion that peels can scar with dark marks, so keep that in mind when getting a peel with a certain strength level. The higher concentrations might exfoliate more harshly, but they also may scar more easily.
Oh by the way, I applied the peel once to the backs of my hands and WOW! I immediately saw results, as in no dark age spots, MUCH smoother skin, and an overall 'freshening'. I'm going to apply the peel to my arms when I get around to it and see if that has the same affect. As in everything, we all have different skin and different results, but I think peels are certainly worth doing if you're interested in improving aged skin.