If I agree to do quests for a specific faction does that mean that other factions see me as a member or supporter of that faction and thus preventing me from doing quests for other factions?

Sorta. EG: Siding with NCR will block Legion missions and Powder Ganger missions.

It depends on whether or not two factions contradict each other, like PG's and NCR aren't exactly the best of friends, so siding with one of them might cause a rep drop for the other.

Thx, in other word think abaout who I team with!