Any kind of bleach (even highlighting) is damaging, as well as using flat irons or curling irons.  Highlights are typically done by foiling, and the bleach they use to do it is the same stuff you use all over your head, just in a lower strength (unless you want platinum highlights). The stuff they sell at Sally Beauty to do your own at home is actually what they sell to the beauty schools and I’ve seen it used in salon’s before too (you actually need a cosmologist license to purchase some of it). It all depends on the strength, how you mix it and how long you leave it on but all of it contains some form of bleach and all of it is damaging to you’re hair; which is why they recommend you don’t bleach, or highlight your hair very often; and when you do you have to be sure to always use conditioner. 

There’s a bit of a  guide on it here.