With 54 creatures in each of five decks, you have 270 creature types at your fingertips. The previews show you exactly what you get with each creature: good artwork and the Fate specs for the creature: concept, trouble, other aspects, Fate Core skills, stunts, extras, physical and mental stress tracks, and size.
Most creature types get a single card: one stone giant card, one red dragon card, one ettercap card, and so on. Some get two or three cards for subtypes, such as goblin, goblin leader, and goblin shaman or lamia and lamia matriarch.
If you're using Fate Accelerated, Evil Hat has a Fate Accelerated/Core Conversion Guide that gives you a conversion method. That guide also notes, however, that "it’s not really going to wreck anything for you to just leave them as is, roll what you gotta roll, and stay focused on the players." (It's referring to Core/Accelerated conversion in general, not the creature decks in particular.)
Each card gives the creature a scale rating, which isn't a Fate term. The title card says that scale is "an indication of the creature's power." Whether and how you use the scale rating is left up to you; the cards provide no game mechanics for it. One might quibble with some of their choices, such as giving an ordinary toad (Small and Harmless Amphibian) the same scale rating as a zombie.
Monster stat blocks in other RPG systems tend to focus heavily on combat stats, leaving non-combat elements to the descriptive text. These cards include combat and non-combat elements, fortunately, and sometimes hints on when they'd seek or avoid combat. The drider is a Corrupted and Shamed Elf/Spider Hybrid. The goblin leader has If You Can't Beat 'Em, Hide. A griffon is a Mythic King of Beasts of the Sky and Earth. A manticore has Buy Me Off - Please.
There's no narrative description of the creatures beyond what you see on the cards. You'd take inspiration from the card elements and from your own understanding or vision of the creature. Some of the listed aspects are obvious and straightforward. An eye of the deep Sees in All Directions. A hippogriff is a Half-Horse Half-Hawk Flying Steed. Some aspects are more open to interpretation. A harpy has A Taste for the Exotic as its trouble and What Is That Scent? for one of its additional aspects. A stone giant has What Has Been Hewn in Stone. This is good news if that inspires your group, or bad news if you want a more detailed explanation of what it all means.
Magic-using creatures have magic-related aspects, stunts, or extras. There's no particular system of magic other than using these things to create magical effects. A kobold shaman, for example, has Trap Magic and Minor Illusions as stunts, with brief descriptions of each.