There are two possible ending for Dark Souls: "To Link the Fire" and "The Dark Lord".
These are popularly referred to as "the Fire ending" and "the Dark ending".
To Link The Fire
By linking the fire, The Chosen Undead embraces self-sacrifice, thus revitalizing the land and banishing The Curse of The Undead.
The Dark Lord
By choosing this ending, you become The Dark Lord, The True Lord Of Men, thus realizing the designs of Darkstalker Kaathe.
As The Dark Lord, you will usher in an Age of Dark... an Age of Men, and your Lordship shall prevail as the supreme sovereign.