Is Master Chief a human or some sort of robot/alien?

Solution 1:

Master Chief, Spartan 117, is human. His name is John (no last name is ever given). He was a citizen of the colonies that was kidnapped at age 6, and taken to train for the military (in secret). In his teen years, he received a series of injections that augmented his physical capabilities to above and beyond what any normal human would ever be able to do. Yet he, and all the other Spartans, remain human.

So why not show his face? Well, that actually stems from how Bungie wanted to approach the character. Master Chief was always meant to be someone you could imagine yourself being. They never intended to reveal who Master Chief was, or show his face, because they felt it betrayed their fans ability to place themselves into the character. Now that 343 has taken over, though, we'll have to see where they go with things, and what they decide to do in the future.

Solution 2:

He is a human because if you finish the campain on legendary you see his forehead and its human + you see him as a kid in the opening scene.

Solution 3:

John-117 "Master Chief" was a human, but after going through the spartan program he is no longer marked as a human but as a "spartan supersoldier"

Master Chief was a light skinned with red/brown hair with freckles when he entered spartan training, but As he is now always in armor pretty much, his hair became white and thin, as well as his skin becoming ghostly pale. during Halo 1 and 2 Bungie wanted their hero to be faceless and without personality so he is "easier to get into", However, as the series grew. books were written (official), and other adaptations (movies, anime, comics, etc) gave Chief and the rest of the universe their own personality.

I suggest reading "The Fall of Reach" book. It explains the backstory very well and gives some insight to the spartan program.

here is some more source and reading from the off the wiki about John-117. click me