Card rarity is a general indicator of how rare and/or valuable a card can be. There are 8 possible rarities that a card can be:

The rarity of a card can be found on the bottom right of a card to the right of its card number. (On Problem cards, the card number/rarity information is located on the top or bottom of the card, or on the right or left side if you are reading it from a player's perspective.) 

A foiled card will usually not have the card's non-foiled rarity shown, instead replacing it with the ƒ symbol. The exceptions for this occurrence are Super Rare, Ultra Rare, and Royal Rare cards (which were always foiled until Seaquestria & Beyond, and will display the regular SR, UR, or RR instead of the ƒ symbol) and Promotional cards (which display both the P and the ƒ symbol).