"uwu" is not actually a word, but a text emoticon. If you look closely, you can see that the "u"s represent two little closed eyes, and the "w" represents the mouth of some kind of mammal. The resulting expression is intended to and does indeed generate adorability, so the mammal is widely presumed to be a small dog or a cat. This is not to say that it couldn't possibly be a seal or something similarly appealing.

One should exercise caution in using the "uwu" emoticon without clear irony, especially as a male, as it can have negative connotations outside of weeaboo circles.

Ex. 1 (correct)

Jimmy sends photo of new puppy to claire.

Claire: OMG thats soo cute uwu

Jimmy: Haha yeah wyd later

Ex. 2 (incorrect)

Jimmy: Wanna hang out? uwu

Claire: Hey sorry man Im busy forever

A weeaboo is a person from a western culture who is obsessed with Japanese culture at a superficial level. Being around this type of person is normally uncomfortable for people from both Japan and the west.

They are characterized by their extensive knowledge of various anime, insistent use of Japanese phrases ("Kawaaii! etc) and unsettling interest in suggestive drawings of girls/childlike women.

Their understanding mostly comes from imported Japanese pop culture seen through the distorting lens of their own experience of social isolation in a western context, and they are not to be confused with people who have a genuine in the culture and history of Japan.