These are the stats creatures will add to your characters stats when having them as active pets (taking them out as well). Pets will gain stats by rising in levels. You can raise the level of your pet by collecting experience with every gathering-action (plucking, cutting wood, mining) in the wilderness (not on your farm though) or per chance from time to time by feeding them. It will also gain experience and levels when being close to at least two compatible pets of other players.
"Inventory" means, that some pets have an additional inventory of 5 or 10 slots you can use. But you can not activate stuff in their inventory directly and will have to move it to your characters inventory first.
"life" adds to your characters health-points
"action" adds to your characters action-points (stamina)
"energy drain" adds to the damage your character deals by attacking creatures
"defense" adds to your characters defense (normally only gained by the equipment your char is wearing)
"critical hit" adds to the chance of critical hits your character can deal (per chance) in a fight
"dodge" will add to your characters chance of dodging attacks of creatures
"char-level" means that you can equip the pet (and its card) only after reaching the mentioned experience-level with your character
After getting a pet-card you can equip it by right-clicking on it in your inventory. Press the key "e" or open your character-stats and click on the button at the right bottom corner (house) to see the stats of your pet. These stats will be added to your characters stats as long as you take out your pet.
To take the pet out click on its small (yellow) card-icon down below in your pet-window. You can equip up to 3 pets depending on your characters level, but you can only take out one pet at a time. The pets you have equipped but did not take out will get hungry too, so beware!
You can feed your pets after taking them out by right-clicking on pet-food in your inventory. Also you should stroke/pet your pet often enough to raise its comfort-level and also clean them up (getting pet-jelly) when they need to take a crap (they will show this with an icon). When your pet gets very hungry it won't leave you, but will try to attack you instead and make comments about being hungry.
But if you don't pet/stroke your pets regularily and their comfort-level will sink to zero they might run away forever! Their comfort-bar will drop as well when being near "uncompatible" pets of other players. So you'd better not forget to keep an eye on the comfort-bar of your pet in your pet-window or you might lose your pet for good if it feels too uncomfortable.
Also please note that you can unequip your pet by taking it out, right-clicking it and clicking the icon that looks like two cards. But your pet will lose every experience it has gained above its current level.