I used to think spiderman shot webbing out of a webgun on his wrist (like the toy ones you could buy) then I actually saw the movie and they seem to come out of some kind of silk gland??
Yeah, that's a movie thing. In the original comics Peter Parker invents them himself. In Marvel's ULTIMATE universe, he still invents them, except the web material is based on some of his father's research. </geek>