How do I know what cards or sets are legal to play with?
It depends on what format you want to play.

Casual

You can use whatever cards you want, as long as your friends are alright with it. This is informally known as "kitchen table magic", at least in my area. Some casual formats are are so popular that they have their own rules, such as Commander. See the link at the bottom for more information.

Standard

Prior to 2016, you could only use cards from the latest two expansion blocks and any core set(s) directly succeeding them. Starting in the Spring of 2016, you can only use cards from the latest three expansion blocks. For those unaware, a "block" is a set of expansions that are released sequentially and revolve around a single story or plane. More details on this format here.

Modern

You may use cards from any set since 8th Ed. Some cards are banned.

Legacy

You may use cards from any set. Some cards are banned.

Vintage

You may use cards from any set. Some cards are banned. Others are restricted to one copy per deck.

All Constructed Formats

The following rules apply Standard, Modern, Legacy, and Vintage.


Deck must contain at least sixty cards
With the exception of basic lands, decks may not exceed four of any card
Cards with a silver or gold border only are illegal
Decks containing a mixture of Alpha and non-Alpha cards must be played with opaque sleeves
A card from an older set is legal if a card with the same name was printed in an otherwise legal set


You can learn more about these formats, as well as some that I did not cover (such as Commander), by visiting Wizards of the Coast's website.