These are two eyes.
This is a nose.
And this is a mouth. Which gives ...
The basic happy smiley! Vary to suit additional needs.
... not too happy at all ...
... silly, tongue hanging outside ... And so on ad infinitum ... Not an expert myself, but most are pretty obvious anyway, and a great deal aren't used too often. Some people like to skip the nose too. Enough of this, move along!