https://www.thehandstand.org/archive/jan2002/EdcellJan2002/rezeq/ - this article explains it pretty well. The historical relationship is pretty consistent. The PLO was multi-ethnic and pan-Arab socialist. Hamas is an (Sunni/Wahabi) Islamist sectarian group that breaks the unity of Palestinian resistance, in theory - though it seems that the Palestinian street has just moved in that direction. The PLO became corrupted and captured, and now is the collaborationist PA.
The real threat, at this point, is folks like Marwan Barghouti (currently being held without charges, in Administrative Detention), and left groups like the PFLP. They undermine the Zionists' preferred (fraudulent) narrative about the conflict being one of Islam vs. Judaism - ala Huntington's "Clash of Civilizations" thesis. The argument becomes - 'we can't have a 2-State Solution - because there's no one who wants peace, to bargain with, on the Palestinian side'.