You will encounter a variety of enemies in the different modes of Halo 4. They will primarily be split into two categories based on their origins. The Covenant were the primary antagonists of every Halo game up to Halo 4. While they appear in Halo 4, it is unclear the role they will play, their importance, or even how they found the planet that you've stumbled upon.
The second group are the Prometheans. Not a lot is known about the Prometheans themselves except that they appear to have been derived of Forerunner design. They are appear to be atleast partly machine but whether they are completely inorganic, organic with heavy modifications or primarily organic with an inorganic consciousness is unsure. What is known is that they come in a wide variety of forms, pack completely new styles of weapons that appear to be of Forerunner origin, and look like they were intentionally designed to look fearsome (and fight aggressively).
It has been noted that at some point, the Prometheans and Covenant will both be fighting against you as a team.