The Best method to do this? (not sure of the term)

[SabbyKat]

i apologize for the rather.. poor title, but i am fairly clueless as to what the correct term is for what i'm thinking about.

that would be what is the best method to link a gallery, vbulletin, and your normal webpages into a single login? i believe is some command user thing, i really can't remember >_< i understand bridging between two things being fairly simple, but i mean a site-wide bridge, merging everything together nice and neatly.

if someone could give the correct term aswell, i could place my question down more appropriately aswell haha -.-

thanks :P