The term 'try square' is a bit vague, and it is fair to view it as a clarifying term; just as one might say 'spinning top' to specify what sort of 'top' one is refering to, a 'try square' clarifies that you are talking about the tool, not the shape, not the public square, and not Bob from engineering.
But usually a try square is a type of L square used for carpentry. The archetypal try square is an L with the short side made of wood and the long side from metal. The metal 'blade' is narrow, while the wooden 'stock' is much wider, meaning that when you lay the blade flat against a surface you can still get a square measurement with the overhang of the stock; this is a useful straight edge for marking wood at right angles to an edge. It is also, of course, used for making sure that boards, joints, and etcetera are indeed square, but people do not generally expect a try square to be high-precision.
Modern try squares may be made entirely of metal, and if they have a wooden stock they will likely have a metal core running down the stock, the core being continuous with the blade (i.e., it's just a simple metal square with a wooden sheathing on the shorter leg). The most common length for modern try squares is 8 inches; historically they went up to 12 or 24 inches, but the stock gets bulky and the tool unwieldy at larger sizes, and these days larger squares are usually simple aluminum L squares. However, a longer aluminum square with a thick stock and a thin blade is still called a try square.
Many try squares are also miter squares, often referred to as try miters. In this case the stock is cut so that it joins the blade at a 45 degree angle, meaning that if you turn it on its side and rest the top of the stock along an edge, the blade will cross the work piece at a 45 degree angle, making it easier to mark cuts for miter joints.