Random sampling: a method of selecting a sample from a statistical population so that every sample that could be selected has a predetermined probability of being selected.
(2010). Retrieved from The Free Dictionary website:
http://www.thefreedictionary.com/random+sampling

In other words, random sampling ensures each sample has an equal chance of being selected.

Example image:
http://faculty.elgin.edu/dkernler/statistics/ch01/images/strata-sample.gif



external image Screen-shot-2012-09-18-at-10.01.50-AM.png This image represents random sampling by picking names out of a hat. There's no way of knowing who or what you're picking so the result is unpredictable. (Dillon Hernandez)
http://blog.surveymonkey.com/wp-content/uploads/2012/09/Screen-shot-2012-09-18-at-10.01.50-AM.png Link to image ^