Card skills will play a large role in which ones players choose to attain. They also play a huge role in the value. For example the recently released cards Apsara vs Chimaera. Both are defensive orientated cards but since Chimaera offers a Big Hit to Demons Attack it is worth significantly less. Apsara offers a Big Boost to Gods Defense, the player can control which cards are in his/her defense deck, but cannot control the alignment of those attacking.
The way this chart reads is it breaks the percent buff/debuff offered based on whether it is a small/medium/big/great/massive/absolute. The Affects column refers to whether or not it is a type 1 (man/god/demon specific), All, or Self and the percent buff/debuff changes based on that information. Finally it states whether or not the skill is attack related, defense related, or both. For example, Inugami offers a Big Attack/Defense Boost to Demons, this means she will apply an 8% increase. Nightmare offers a Big Attack Boost to Demons, this means she will apply a 13% increase.
These numbers are applied before skill boosting (leveling up a skill through enhancement). The chart is also not complete since some percentages are so negligible they don't matter (small) or because they are not seen on cards currently (self attack only buff).
By enhancing a card with other cards that have skills a player has a chance of increasing a cards skill level. Each skill level gained increases the potency of the card by 1% capping at skill lvl 10, or 10%. Most player achieve this by buying high normal cards and enhancing their card with nine of them in order to gain a skill level. The percent chance of gaining a skill level is changed based on the rarity of the card being used as the enhancer, the number of enhancers, and the rarity/skill level of the card being enhanced. For example, if a player attempts to use nine high normal skilled cards to enhance an SSR the percent chance of increasing from lvl 1 to lvl 2 would only hit around 12%. To increase from lvl 5 to lvl 6 would be near 2% if that.
The card Devil Queen gives a 50% chance to increase a cards skill level no matter what. These are often used to enhance high rarity cards to save on time and money.