CMC is an awesome online TCG, especially if you have read the webcomics by the maker of the game.
The gameplay is easy to learn, but offers enough variety to stay interesting. The best part of all, it's completely free!
A lot of the gameplay revolves around your mana pool. Each turn, you gain 1 of each element of mana (Gray, Dark, and Light). This number can be hightened via effect cards called Generators. When cards are played, mana is decreased from the pool.
There are three types of cards, creatures, effects, and spells. 5 creatures can be in play at once. These have an attack and life value, and can attack opponents creatures or directly attack the opponent. They are then 'dizzied', which stops them from doing anything till your next turn. Undizzy creatures can block attacks from opponents creatures, taking the damage themselves.
Effects have many different...effects. They are placed on the field, and can do many different things, from generating mana, to summoning a field full of faries.
Spells can be played at any time, and have many different possibilities.
The best way to understand it is to play it, or to observe ongoing games. To get started, make an account at
http://www.cardmasterconflict.com If you want an easy time, don't choose the mech decks at first. They have extra game mechanics, and take two colors of mana.
They have a wiki at that site that can explain it much better than myself.