Otaku is something like official term, weeaboo is 4chan terminology, I must be M when I enjoy being called that because it's just something noobish 4ch community thought of and isn't official =D
Well there's a difference between weeaboos and otaku. Some people swear up and down that they're the same, but the do have differences...although they are both originally derogatory. The phrase otaku was created as an insult to those who were obsessed with anything (as in, to completely unhealthy levels). Though it's often used for anime and manga, it can be referring to anything, and it's rare that one won't be offended by being called it. In the West, though, it's undergone meaning evolution. Here it basically means X-geek, "X" for anything but are mostly used to refer to those who are into anime, manga, and things of that nature. Heck, here people actually call themselves that affectionately, but on its own it has little to do with Japan, it just often refers to an interest in Japanese pop culture (but doesn't have to).
Weeaboo technically isn't inherently derogatory. It just refers to someone who wants to be Japanese. That in itself is pretty questionable, because it already gives implications of misinformation on the part of the person in question. And...that's pretty much it. The problem is, a lot of times (and most of the time) the reason the person wants to be Japanese or live in Japan is because of what they see in anime and manga. That's a large reason why the two are used interchangeably. They often have a warped view on Japan or Japanese culture thanks to the exaggerated representation that anime often presents. These are the guys that will sprinkle popular Japanese words into sentences for no reason or use honorifics in average conversation, or sometimes they'll act the way they see in anime or the like. As such, the interest they have in Japan is often fiscal or unrealistic, based off of some collage of gathered fictional information.
I'm not the type to try to tell people not to be themselves, and heck just because you do some eccentric things that could be considered weeaboo doesn't necessarily mean you are one. Like I, for example, listen to predominately Japanese music. It's not weeaboo, it's just a preference, and I know some people use honorifics just because they like to use honorifics. But the problem with being a weeaboo and acting on your information is that one mocks the very culture (and race) that they're trying emulate.
