Henry is right: Google is your friend.
If you look, you will find that this question has been asked, and answered so many times, that many people are tired of looking it up and posting it for you. Do yourself a favour: Learn to research your own basics. It is a lot quicker in the long run than asking others to do it for you - and a lot more satisfying.
To get you started, copy your subject line and paste it into a google search. Add "C#" to the end to limit it to the language you are interested in. Search. This gives
nearly 1/2 a million hits[
^]
Now, why didn't you do that?