PLO is a was originally founded in 1964 as a convergence of the main Palestinian resistance groups, the Democratic Front for the Liberation of Palestine, the Peoples Front for the Liberation of Palestine, Fatah and a few others. It's military wing was originally a group of more or less regular army formations, intended to operate sided by side with other Arab armies in wars with Israel.
The PLO also founded the PNA (Palestine National Assembly) as a represntative body, aside from the Central Committee of the PLO) The PNA became the legistaltive assembly that ran the Palestine Authority.
The difference is, at least in theory, the PLO is a coalition of groups organized around forwarding Palestinian rights, the PA is nominally a state structure run by a legislative assembly of elected officials.
Its murky and confusing, but my feeling is that is a result of the nacent nature of the Palestine Auhtority as a state structure. Were the Palestinians to achieve statehood, it is likely that the PLO would break into its various factions, who already compete for positions in the PA.
Fatah is the largest faction and so it usually leads the PLO, and consquently will likely lead the PA, if its candidate wins the election, if that happens.
[ 24 November 2004: Message edited by: Cueball ]