Usually, to have a little more fun in Counter Strike, people will do something called "Knife Fighting"
This is a one-on-one fight using the melee weapon you are given at the start. Most people tend to hate it, because they don't know how to get the kill they need.
Sometimes this isn't the case, and you just want to kill a person who doesn't notice you, and you want to take advantage of that.
These are some easy tips to score that extra kill.