The Extractor is a building placed on the sea floor, usually on top of ore deposits. The extractor automaticaly mines out the ground under it, putting the resulted item into the Extractor's 2x2 inventory. You can find out what percentage of the ground under the Extractor that it will collect by opening the Extractor's GUI then looking at the percentages on the right. You can have more than 1 extractor on the same ore deposit.
There are two types of extractor: the regular and the overpowered.