Right Angled Triangles
A right angled triangle is (you guessed it), a triangle
that 
The little square in the corner tells us
that it is a right angled triangle (I also wrote 90°, but you don't need to!) 
The right angled triangle is one of the most useful shapes in all of mathematics!
(It is used in the Pythagoras Theorem and Sine, Cosine and Tangent for example).
Two Types
There are two types of right angled triangle:
Scalene right angled triangle
One right angle 
Isosceles right angled triangle
One right angle 
Example: The 3,4,5 Triangle
The "3,4,5 Triangle" has a right angle in it. (Draw one if you ever need a right angle!)
It has no equal sides so it is a scalene right angled triangle
