Yes, brass is an example of an alloy. Another common one is steel, which is an alloy of iron and carbon (with a few other things as well, depending on the type of steel).
To understand alloys at a microscopic level, there are actually TWO levels at which you must understand metals in general, and then expanding the idea to understand alloys as opposed to pure metals isn't that much of a stretch.
First, at an atomic level. Metal isn't made of molecules in the same sense that many other things are (ie, one oxygen atom and two hydrogen atoms sharing electrons just with each other to become a water molecule). Atoms of a metallic element CAN form covalent bonds and become a molecule (two iron atoms and three oxygen to make rust, for example), but any time you see a solid piece of metal, its atoms aren't grouped up into molecules. Instead, the atoms all loosely share the electrons with all the other atoms around them in what my professors sometimes called an "electron soup". This loose sharing of electrons that allows electrons to move anywhere in the entire piece of metal is why most metals conduct electricity very well.
Next, at the "grain" level. The atoms of a metal (pure or alloyed) will fall into some kind of 3D arrangement depending on the metal, what its alloyed with, how its been tempered, etc. This arrangement is called the metal's "lattice structure". The lattice structure is usually some sort of repeated pattern, but instead of it being repeated perfectly through the whole piece of metal, there are crystals or grains in the metal (still microscopic) in which the pattern is repeated correctly, bumping up against each other, and the pattern doesn't repeat correctly across the boundary from one grain to the other. Sorry, this is a bit easier to draw than describe verbally.... The physical properties of the metal are determined by the lattice structure of the atoms and by the grain structure of the metal.
When you alloy a metal, you're mixing in some other element into the lattice structure (3D pattern) of the metal. There will be atoms of the other element spread more or less evenly throughout the piece of metal in between the atoms of the "main" metal. If you're using a non metal, like adding carbon to iron to make steel, you have to add a small enough amount of it that you don't interfere with the "electron soup" of shared electrons between the metal atoms. If you're mixing two or more metals, you can use whatever proportions you want. Regardless of whether you add a non metal or another metal, you will change the lattice structure that the atoms fall into. This will affect the size and types of grains the atoms can form, which will change the physical properties of the piece of metal.