Critique is feedback on how you can improve. I appreciate it, and do not think I get enough of it, even if it is phrased as a critisism, which most people here are pretty good about.
To add to what meggierenee said, a critique offers constructive feedback as well as encouragement. A proper critique should point out both flaws in a friendly way and strengths. It need not be indepth, just a comment on something you note may be off a bit or could use work on and this should be followed by pointing out a strength or a bit of praise. One should always sandwich a negative comment between two positive comments or at least end on a positive note. This leaves the critiqued with a better sence of well being and encourages them to work through their problems. I love when I get a critque. I take some to heart and others I appreciate but for one reason or another decide against them. Almost always though, the critique helps me improve, pushes me to be better and in turn allows me to share great art with the community.
A critique is constructive feedback on your art. I haven't had any critiques on dA, but I have found critiques to be extremely helpful in my college art classes.
How do you feel when someone criticizes your artwork?