To expand on Mnemosyne's excellent answer, it's not just creatures. With certain exceptions (below), you are only allowed four copies of any nonbasic land card in a deck. The 4 of rule only checks the actual name of the card. So I can have 4
Dragonlord atarka and 4
atarka, world render in my deck.
Also note that the Plainswalker rule differs slightly from the legendary rule. With a legendary, you cannot have more than one legendary permanent with the same name. So I can have a
dragonlord atarka and an
atarka, world render in the file at the same time because, although they are both Atarka, the cards have different names. Planeswalkers, on the other hand, you cannot have more than one of a certain planeswalker (Mem's example of Chandra)
The 4 card limit exceptions are if the format you're playing has different rules (Commander, singleton, etc.), and if the card, itself, allows you (
Relentless Rats,
Shadowborn Apostle)