When two lines intersect and form four equal angles, we call each one a right angle. A right angle measures $90^\circ$. You can think of a right angle as a quarter turn in one direction or the other.

An angle in which the two sides form a straight line is called a straight angle. A straight angle measures $180^\circ$. A straight angle can be made by putting right angles together. You can think of a straight angle as a half turn, so that you are facing in the opposite direction after you are done.

If you put two straight angles together, you get an angle that is $360^\circ$. You can think of this angle as turning all the way around so that you are facing the same direction as when you started the turn.