Arbitrary triangles are categorized according to their angles.
All angles of an acute triangle are less than 900.
One of the angles of an obtuse triangle is more than 900.
Right angled triangle
One of the angles of a right angled triangle is precise 900 (a right angle).
The right angle is usually marked with a small square.
There are a lot of formulas only applying for right angled triangles. Therefor we have a site dealing with only this.