A negative term directed to anyone overly obsessed with Japanese culture to the point where they become annoying.Used frequently on the image boards of 4chan.
Most weaboos are uneducated about their obsession of choice and are often noobs who are overly zealous, trying to impress others with their otaku knowledge. Another trait of a weaboo is their desire to "be Japanese".
While the two terms are often fused together, weaboos are very different from anime fans or enthusiasts. A fan may be just as enthusiastic and knowlegable about Japanese culture, mainly anime, but they neither boast about their knowledge nor call themselves otaku(because of its known negative connatation).