An angle is the amount of curve between two lines meeting or crossing at a point.
∠ is used to denote an angle.

In the figure above, lines AO and OB meet at O to make the angle AOB or ∠AOB.
O is the vertex of ∠AOB.

Angles are measured in degrees using a protractor.
To measure the angle EFG between two lines EF and FG drawn below:

1. Place the base line of the protractor over one of the lines, say, FG.
2. Place the centre of the protractor on the vertex i.e. F.
3. Read the scale starting from 0° at FG to where FE crosses it. That reading is ∠EFG.
   
   In our case, ∠EFG = 55°

Another example:

Following are some special angles:

1. Right angle
   When two lines form an angle of 90°, the angle is known as a right angle and the two lines are said to be perpendicular to each other.
   
   
2. Acute angle
   Angles less than 90° are acute angles.
   
   
3. Obtuse angle
   Angles more than 90° are obtuse angles.
   
   
4. Straight line
   When two lines meet at 180° they make a straight line.