What is Pauper?
Most basically, Pauper is a Magic: the Gathering format, where only common cards are legal.
If a card is legal in any printed set, it’s legal in pauper. ie: Kird Ape is common in Arabian Nights and Revised, but Uncommon in 9th Edition.  Legal in Pauper.
How do you know if a card is common?
Just check Gatherer. Gatherer is the official source of rarity information for Wizards of the Coast.
The banned list for pauper currently consists of one card: Cranial Plating and the official Wizards banned list can be found here.
Where can I play?
While most pauper is played on Magic Online but if you’re near Syracuse, NY we have a paper pauper league that meets every Wednesday at noon at Play the Game Read the Story in Carousel Mall.