The Collector is a large, mouth-like organism located in water near the Broken Land, that can digest anything that falls into its giant maw (as evidenced by the fact that it can apparently digest The Ark unless Will takes it out in time).
It is able to spawn shrimp-like organisms that apparently help in its digestion; these organisms are about as large as Will himself. They can also shoot some kind of digestive juices at Will that can cause him to take damage.
At the center of The Collector is a large mass of bubble-like stuff that, when shot, causes The Collector to take damage. Destroying all of these bubbles makes The Collector open up its mouth, which allows Will and The Ark to escape.
The Collector appears to have several horn-like appendages attached to its mouth and scattered about its body. It also has several vents at its sides that spawn the shrimp-like organisms. The Collector is a gigantic being, making the already massive Ark look puny in comparison.
The Collector is probably named because of its giant mouth, which would be able to "collect" a great many things at one time, before it digested them for food.
the collector may or my not be a reference to the Sallac from StarWars