According to the unabridged and authoritarian guidebook for gay, bisexual and straight-curious males,
You Might Be Queer by Anonymous, a top is the male who fucks another male in the arse and a bottom is the male who gets his arse fucked by another male.
Culturally speaking, the terms 'top' and 'bottom' refer to anal intercourse roles for males. To keep it simple, if you are a male and your arse is being penetrated by another male's cock, you are fulfilling the sex role as a bottom. Conversely, if you are a male and it is your cock that is penetrating another male's arse, you are fulfilling the sex role as a top.
The guidebook I referenced above is either out-of-print or the manuscript was never published.
However ...
Generally speaking, the terms 'top' and 'bottom' are unique to gay and bisexual male culture as they narrowly define sex roles when males engage in the practice of anal intercourse with other males ... regardless of whether the male self-identifies as either gay, bisexual or straight-curious.