  A triangle is a polygon with 3 sides and 3 angles. The sides can be any length, but the 3 angles always add up to 180°

A protractor is the measuring tool used to measure angles. Angles are measured in degrees.

An acute angle – less than 90°
An obtuse angle – greater than 90° and less than 180°
A right angle – exactly 90° The box is the symbol for a right angle. This lets you know it is exactly 90 degrees and you don't have to use a protractor to check it.

Triangle Classification:
Triangles can be classified by Side Length and/ or by Angle Measurement.

Triangle names by Angle Measurement:

A triangle with all acute angles (all less than 90°) is an acute triangle.
A triangle with one obtuse angle (more than 90°) is an obtuse triangle.
A triangle with one right angle (90°) is a right triangle. Triangle names by Side length:

A triangle with 3 equal sides is an equilateral triangle.
A triangle with 2 equal sides is an isosceles triangle.
A triangle with no equal sides is a scalene triangle. If a triangle has 2 congruent sides, then it also has 2 congruent angles.
Notice the picture below. 