A combination square is one of the most common modern tools, one of those that everyone has in their toolbox. It is mostly a square, used for determining right angles, but it has a bunch of bells and whistles to make it super useful for people who don't need a specialized tool.
The square consists of two primary components; a 'blade', which is essentially a metal ruler, and a 'head', which is a plastic or metal clamp that attaches to the blade.
The blade is indeed a ruler, usually either 12 inches or 300 millimetres, although other sizes are available. The quality of the blade is highly variable, but is almost always made of steel. When removed from the head, it distinguished from the other steel rulers one might have around the house by a groove down the center, used as a guide for the head.
The head is where things start getting fancy. The cheapest combination square you are likely to find will have a plastic head with a lock nut to tighten it to the blade, or loosen it to allow it to slide along the blade; the head will have a right angle edge on one side and a 45 degree angled side opposite; and a small, built-in bubble level. The normal ones have a metal head and a removable scriber tucked into the top. The next step up is to add additional heads, most often a center head, used to place the square to scribe a line across the end of cylinder so that it will cross the center point; and a protractor head, for finding angles other than 45 and 90 degrees.
And that's mostly that. You can find combination squares with notches cut for especially accurate scribing, graduations down to 1/64th of an inch, extra chunky heads (e.g., 2+ inches along all three axis) so that they won't tip when left standing, that can double as a a saddle square, that have Cool Retro Wooden heads, and etc. It should be noted that a square of the form of a try square with a movable head and (usually) none of the combination square's bells and whistles is usually called a double square.