Allies, in the generic sense, are countries that are on the same side in a war or which are generally on the same side of international issues. In the case of WWI, then, Russia and Serbia were allies while Germany and Austria-Hungary were allies. It is possible for countries to be allies in one war even if they are not particularly friendly with one another.
In the context of WWI, one of the sides was called the "Allied Powers" while the other was called the "Central Powers." So, if we talk about the "Allies" with a capital "A" we are referring to a specific group of countries. The main countries on the Allied side were France and Great Britain with Russia being on that side until it dropped out of the war and the US joining that side very late in the war. Italy and Japan were also on the Allied side though neither played a very important part in the war.