Collocations - An arrangement or juxtaposition of words or other elements, especially those that commonly co-occur, as rancid butter, bosom buddy, or dead serious.
They are collections of words that occur together in discourse.
Below are two sites to practice collocations in English.
http://www.ccym.edu.hk/english/Hkcee/Collocation.htm
http://www.better-english.com/strongcoll.htm
