A set square, often called a triangle in American English, is a square used in drafting and technical drawing. It is simply a right triangle.
There are two common forms, one being a frame in the shape of a right triangle, giving you the inner ages of the frame as an additional guide; the other is a solid triangle, which allows for a half-circle protractor to be inscribed on it. Both of these will usually have measurements on the edges so they can be used as a ruler. The most common set square is a 90-45-45 triangle; when this triangle has a protractor on it, it is called a geodreieck. You will often get a 90-45-45 triangle together with a 30-60-90 triangle, giving you a wider range of angles; the two can be paired to give 15 degree and 75 degree angles as well.
Set squares are commonly used with compasses and rulers in geometry classes, and with T squares and drafting tables in technical drawing.
It is very tempting to call these 'triangle squares', and I do, but this pains the mathematicians, and that term actually refers to a different type of square that is a triangle. A triangle square (a.k.a. a speed square or rafter square) is used by those who construct things in three dimensions; it is a metal 90-45-45 triangle with a 'foot' running along one end allowing it to rest along, and square around, corners.