Nothing. It's just a way of showing that you agree or disagree with what someone had to say. It doesn't affect anyone's Xper in any way. It may have an effect on how a Most Helpful Opinion is automatically chosen by the system (if the QA waits too long and never chooses it themselves), buut I'm really not sure what all goes in to that.
Nothing happens. It's simply a way to express agreement or disagreement with a particular users' opinion. However, some users here abuse it and just use it to troll or down vote the people they don't like.
Hi, It's more for users to get an anonymous poll on how others reacted to the opinions and if they liked it or disliked it. However, on your profile page there is a quotation, one that is picked from the top 10 up voted opinions you have posted.