A skill card improves the performance of a skill in any of several possible ways. Enchanted skill cards enhance the benefit. The highest level of enchantment is +10.
Adding a skill card to the carried inventory of a character that can use the skill named on the card. Once in the carried inventory a skill card is equipped by double clicking on the skill card. You'll see the letter E appear like a watermark over the icon for the skill card and if you check your character's skill inventory (Alt+S) then you should see that the skill description now has a +1 in front of it.
Skill cards are dropped by defeated mobs and can be traded so if you are tired of slaying mobs hoping for a drop then visit the Auction House or browse blue selling shops to look for bargains.The buy back price at an NPC shop is not a useful guide.A common scam is to set up yellow Buying shops to buy these cards from inexperienced players for a deeply discounted price.Be warned! Don't sell or buy an item without first thoroughly comparing prices at the Flea Market or at the Auction House.