The main opposing force of a story. Normally the antagonist is a villain but other forms of fiction may have them as heroes or indifferent characters.
Most fiction displays the antagonist as the complete opposite of the protagonist or the "What If..." style. The antagonist is normally viewed as the big bad.
Two Sides of a Coin
While the protagonist will stand strongly for one issue the antagonist will stand for the other side. These type of antagonist challenge the hero at every cross-point, almost like a rival. The two opposing forces are so radically different they neither can understand the other (sometimes it is because one is insane).
This style is when the antagonist resembles the protagonist in very single way except for an important aspect of their character. This may show some significant ideas or actions between the two. Normally the antagonist has wrong or misguided idea. If so they will either get a redemption by learning the protagonist's ideals or die still believing their lost cause.
Other times these antagonist are just like the hero except at on crossroad of their life they choose the wrong path. It could have been accidental or on purpose but it will change their life forever. If it is this form then that choice eventually led that person down a dark path. These kind of antagonist help the reader distinguish that had the protagonist chosen a different option then their identity may have slowly drifted towards the opposite path.
One of the things that make antagonist so popular is the fact that they can push the protagonist towards their edge. If the protagonist is a hero then they may be pushed into preforming darker deeds.