Criticism is a touchy subject because creative people are sensitive types who get strangely offended when you call their work "excrement" and suggest it be burned.
1. Professional Criticism
When most people talk about "critics" they're talking about professional critics, whose job is to write movie reviews that you disagree with, or restaurant reviews that you disagree with, to name a few examples. The function of professional critics is to make us feel good about ourselves for being smarter at evaluating movies than an official critic.
It's true, you can't argue with the numbers. If you were to take a list of movies and rate them either good or bad, you would find at the end that you were 100% correct and that every movie you rated "good" was a good movie, whereas a professional critic would be lucky to bat .500 on that same list.
So he's getting paid to come up with his opinions, while you, a lowly layman, are by some strange coincidence way more accurate than he is. When you're having a tough day and feeling down on yourself, that can feel pretty good. So that's what professional critics are for, so you can either feel superior to their snobbery in preferring artsy impractical things, or superior to their crudeness in preferring lowbrow trash.
2. Constructive Feedback
So you're working on your novel and you want people to tell you how great it is, and give you ideas to make it even greater. You might pass some copies to your friends or post it somewhere on the internet, and ask for feedback.
A lot of the time people misunderstand and tell you what's wrong with your work and give you suggestions to improve it. Seriously, how is that supposed to help? I guess people are just jerks sometimes.
Seriously, though, you will get all kinds of responses, from helpful people who have good suggestions to well-meaning people with bad suggestions to assholes who put you down but do not have any suggestions. Since you can't seal yourself like a hermit and avoid all criticism, you are just going to have to learn to sift and filter all this and smile politely at everyone, since you did ask for it.
3. Personal Criticism
Sometimes people will criticize you for very personal things, like the fact you are 400 pounds, or that your mother sleeps around with many men. This is actually worse on the internet than face-to-face, which is puzzling because while internet anonymity means people feel freer to say these things, internet anonymity also means that those on the receiving end must know their accuser is pulling this all out of their ass since all they can see is a username and some words.
Nevertheless, people get very angry about being told that they are fat, or smelly, or have sex with animals, by an accuser who can't see or smell them and had no way of seeing what happened with that goat behind closed doors. (They were just talking.) If these things constantly upset you, it might be worth exploring why.
This is when people try to find something other people care about and then say bad things about it in attempt to get them upset, and get attention. For example, walking into a Star Trek convention and telling everyone that Star Trek sucks.
People don't often do that in real life because as weak and harmless as Star Trek conventioneers generally are, they can be dangerous due to their sheer numbers. However, on the internet, many trolls feel comfortable logging in to a Star Trek forum or posting in a Star Trek thread to talk about how Captain Kirk gratifies anonymous men in bathrooms.
Trolls don't care about you or the subject you love, which they are attacking. They only care about themselves and the gratification they get out of the reaction. So it's only polite to return the favor and not care about them either. This is best done not by writing paragraphs explaining why you don't care about them but by actually not caring.