A card has three main stats, its attack in red, health in green and cost to deploy on the board in white. With the exception of Effect Cards which just have an ability, and cost. A picture of the character or object in question is displayed in the top centre portion of the card and their ability in the lower central portion. Cards also have a rarity, for example a white heart on the card means it is common, blue is rare, purple- epic, yellow- legendary and red Determination. There is also a shiny/gold version of every card. This doesn't have different stats, however can be disenchanted for more dust than its usual counterpart as it is a rarer version of the original card. Shiny/gold cards can be distinguished by the rainbow color replacing white areas of the normal version of the card. The rarer a card gets the higher the chances of you receiving the shiny variation. You can also craft shiny cards for an exorbitant amount of dust. Characters in cards are not limited to those you fight in Undertale. For example, Burgerpants has his own card yet you can't fight him in game.
Below is a list of all cards found in undercards, a link will bring you to a another page with greater detail on the card such as tactics for their use and clauses that may not be obvious at first. Currently there are 126 cards in game excluding cards summoned by other cards.
Press ctrl+f on PC to search for a specific card
Dummy, Snowman, Annoying Dog (summons Dog Residue), Froggit, Migosp, Moldsmal, Monster Kid, Loox, Mad Dummy, Moldbygg, Napstablook, Shyren, Temmie, Vegetoid, Whimsun, Chilldrake, Doggo, Flowey, Ice Cap, Ice, Lesser Dog, Memory Head, Migospel, Moldessa, Parsnik, Snowdrake, Snowdrake's Mom, Vulkin, Aaron, Alphys, Burgerpants, Jerry, Madjick, Pyrope, Dogamy, Dogaressa, Mettaton, Reaper Bird, Whimsalot, Woshua, Asriel, Endogeny, Final Froggit, Lemon Bread, Mettaton Ex, So Sorry (summons Doodlebogs), Toriel, Gaster (summons Gaster Blasters), Gyftrot, Muffet, Sans (summons Gaster Blasters), Tsunderplane, Astigmatism, Greater Dog, Royal Guard 1, Royal Guard 2, Knight Knight, Papyrus, Undyne, Glyde, Asgore, Asriel Dreemurr, Frisk, Chara, Nacarat Jester, Sad Customer, Muffet's Pet, Grillby, The Heroine, Rock (creates Pebble upon death), Everyman, Mettaton NEO, Ice Wolf (summons Ice), Glad Dummy, Small Bird, Spider, Gaster Follower 1, Gaster Follower 2, Gaster Follower 3, Goner Kid, Omega Flowey, Janitor, Onion San, Clam Girl, Catty, Bratty, Clam Boy, River Person, MTT Fountain, Sad Dragon, Echo Fish, Dog House, Diamond Boy 1, Diamond Boy 2, Politics Bear, Gift Bear (summons gifts), Cactus, Crazy Bun, Bunbun (summons Bun), Big Bob, Lamp, Snow Poff, Scarf Mouse, Garbage, Trader Temmie, Nice Cream Guy, Receptionist and Oni.