Etc, etcetera, et cetera. It comes from the latin "and the rest". (Ceteris paribus, the famous economists phrase, meaning "taking the rest as constant, equal" has the same root). That means, whenever you are saying:
I'm interested in buying apples, pears, potatoes, etc.
You are really saying
I'm interested in buying apples, pears, potatoes, and the rest.
Those enumerations need to have a final comma in english, as I previously commented in here:
http://www.irosetta.com/questions/4/when-to-use-commas-with-and-and-or/18#18
Also, on everyday language you tend to make a longer pause there, so the comma is further accentuated. For example, in spanish you wouldn't use a comma for:
Querría comprar manzanas, peras, patatas y fresas.
But you would write this:
Querría comprar manzanas, peras, patatas, y demás.
And hence you say:
Querría comprar manzanas, peras, patatas, etcétera.