How Exactly Does This Happen??? (adsense related)

say i visit a hypothetical site today called bobsbbqs.com . bobsbbqs.com had no ads whatsoever on its website

however now when i visit completely unrelated blogs or websites all i see are ads for bobsbbqs reminding me to go back to bobsbbqs and make that purchase.

does this mean that bobsbbqs has purchased ads with google adsense? i went to googles adsense page and it seems they r only selling "Adwords" which is not the same thing as what i saw..

how exactly does this work?