From M:TG Archive, a library of Magic: The Gathering information

Preparation for a game as the players choose which cards to include in their decks. Beginners typically start with only a starter deck; but, over time, more cards are added to the player's stock through purchases or trading with other players. In casual play, the allowed card sets are agreed informally. Due to the many possibilities, two players seldom play with the same decks.

The standard deck size for play is sixty cards, as a minimum total, counting spells, creatures, and lands. Players are restricted to using no more than four of any named card, except that they may include any number of "basic" lands. Some standard formats restrict further the number of allowed cards. When deciding which cards to include, it is often most beneficial to use the minimum deck size, combined with the maximum number of card copies, so that the most useful cards are drawn more often.

The balance of land to spells is the most fundamental aspect of deck building. Most spells have a color, which means that they require lands which will provide that color to be included in the deck. A deck also must have a large enough number of lands so that they are drawn in a timely manner. The ratio of spells to lands is typically in the range of 2:1–3:2.

The five colors each have different strengths and weaknesses. For this reason, it is often worthwhile to play two or more colors, so that the strengths of one compensate for the vulnerabilities of the other. This complicates the interaction of all the components, but results in a more diverse deck.