There are 3 main parts to the Card System in v.42, Gems, Attributes and Cards.
Attributes: These are your core stats: Agility, Vitality, and Intellect.
Agility: This Attribute focuses on Speed and Defense. Upgrading your Agility will give you armor, attack speed boosts, lifesteal, and more.
Vitality: This Attribute focuses on HP and Regen. Upgrading your Vitality will give you increased health regen, and health.
Intellect: This Attribute focuses on Mana and Regen. Upgrading your Intellect will give you increased mana and mana regen.
Attributes are purchased with Gold, which can be done on the fly while in lane by pressing G (the default key) or when you go back to base and open the Shop (this happens automatically, or you can use CTRL+G [default keys] depending on the way you have configured your in game settings).
Each attribute can hold up to 5 Gems, unlocking them at levels 1, 7, 13, 19, and 25.
Spending Gold on Attribute points cannot be undone, so select your upgrades carefully!
Gems: Gems are bonuses that you get for purchasing upgrades in your Attributes. Gems have a variety of Shapes - each one is specific to a certain level of your Attributes. Gems are unlocked at slots 1, 7, 13, 19, and 25 in each Attribute. Each deck can hold any combination of 6 Gems in total.
You can select the slots you wish to have Gems in, based on your play style, hero, and deck configuration.
You can see the Gems in your Attribute categories in game, by hovering over the Attribute:
Cards: Cards can be purchased in game in one of two ways. Most cards are purchased by having the appropriate number of points in the specified Attribute. For example:
The Attribute points needed to purchase a card are located in the top left corner. You can see that the above card requires 4 points in Agility, and 2 points in Vitality to be able to be purchased.
Some cards can also be purchased with Gold, specified on the card in the same location as the required Attribute points:
The information in the lower left of each card shows the effects for the card:
In this example, the Overclocked Droid card has 7 Power, 13 Attack Speed, and a special ability called Frenzy. You can examine your cards in the Collection>Cards menu to read about the different effects of each card. The same information is available in the in-game card shop when you select a card.
To unequip a card, open the card shop and select the card you wish to unequip. This will create a shadow of the card at the bottom of the screen, and you can click on that space to unequip the card.
Cards can be unequipped and re-equipped as long as you have the appropriate amount of points in your Attributes. Cards that have been purchased with Gold can be re-equipped, but the Gold is not refunded.
Keep in mind, there are cards that require Mana to be able to use. You can find the information for how much is required when looking in the Card Menu, or in the Card Shop in-game.