Combined Notes? (Have no idea what to title this)

To preface, looking at the rest of these posts, this is nowhere near as sophisticated, but this seems to be the best place to ask this question, so here goes: Basically, how come if you play two notes, the one that is heard is not the one in between. For example, if you play a C and a D at the same time, a C# is not heard. But not only that, what is heard isn't even any note... It is the same with a chord - how can we hear something that isn't... a note? All general knowledge on the subject would be appreciated, since I've been a musician for a while and this stuff still continues to confuse me!