Many travelers get free checked bags on United; military and their familes, Star Alliance Silver and up (higher status = more free checked bags), Presidental Plus and Mileage Plus Club members get 2 free bags, Mileage Plus Explorer And OnePass Plus card holders get one free bag, and full fare economy get 2 free checked bags.
For the cc holders, the rule now is the primary cardholder and one companion on the same reservation purchased with the cc get the free bag/s benefit; one or two bags each, depending on the type of cc.
The above is true for domestic travel only; for international travel from the US, everyone gets one free checked bag.
All of this information is easily found on United's website.