Common, uncommon, and rare
Many Pokemon cards use a circle for common, a diamond for uncommon, and a star for rare. The symbol is usually near the card number at the bottom.
Holo and reverse holo
A holo card has a shiny artwork area. A reverse holo usually has shine outside the main artwork. These versions can have different prices from the non-holo version.
Promo cards
Promo cards may have a promo stamp or special numbering. Some promos are easy to find, while others become valuable because of limited distribution.
Ultra rares and secret rares
Full art, alternate art, rainbow, gold, illustration, and secret rare cards often attract collectors. Always compare the exact number and artwork, because similar names can hide very different values.
Rarity is not everything
A rare card can still be cheap if demand is low. A popular card can be valuable because players and collectors both want it.