Beings of pure destruction and chaos. In the past, whenever they finished destroying one dimension, they move to the next. The 3 eldrazi titans are, from left to right, Kozilek, Emrakul, and Umalog. (or, as they were previously known and worshiped as on Zendikar, Kozi, Emrak, and Uma. All 3 have the ability to control hordes of smaller eldrazi, ranging from the size of a wolf to 40 feet tall. Their minions take on the apperance and powers of their masters. If they attack your planet, then you should probably just run for your life and pray for mercy. On second thought, running is useless, so just pray for mercy. Except that their more powerful than any god, so forget that.
Kozilek, also known as the butcher of truth, has the ability to warp reality around him. He changes wherever he wlks into a strange, chaotic place where the world itself is transformed into a sieries of strange, geometric squares. His minions, like him, are heavily armored and have eyes in strange places, such as the limb joints or scattered randomly around their body. They all have black shards of an unknown substance scattered around their body which are capable of cutting through any substance with ease. However, they do not use them for weapons, and no one knows what they are for.
Emrakul, also known as the eons torn, is the strongest of the 3 eldrazi titans. She apears as an island uprooted from the earth, with black tentacles dangling underneath. Unlike the others, she has very few eldrazi minions, but the few she has are powerful. Also unlike the others, she does not leave a trail of destruction but instead turns living creatures into her minions (at least, that's what the theory is. An in-depth study of the eldrazi is impossible.) She warps reality to an extreme, including gravity loss and reversal. It is impossible to kill without a large army and several plansewalkers.
...Or, you can attack him with 15 squirrels. (inside joke).
Umalog, also known as the infinite gyro, is the embodiment of hunger. wherever he passes, the world becomes grey and lifeless, made of ash and dust pillars in the shape of the place he passed over (for a time). His spawn are equily hungry, and wherever they pass also becomes bleak and deviod. Like their master, the eldrazi of umalog's lineage have a blank, bony face that hints at a mask or helmet, but there are no eye holes.