Somebody please explain why pres. polls are 2-10 points different

Somebody please explain why polls are 2-10 points different

Yesterday NBC Nightly News reported a new (WSJ/NBC) poll having McCain behind by 10% with Obama at 52%/McCain at 42%. Now a new AP Poll puts Obama at 44% to McCain 43% on the very next day. What is going on? Is any poll reliable? Is this race over? What is causing the swing?