A piggyback program is a program that you don't want that gets installed when you install another program that you want. Usually you are asked if you want to install the piggyback program and given an opportunity to skip the install. I find these programs on computers everyday. Often they are toolbars and present themselves in Internet Explorer as shown below.
These programs slow down your computer and are generally a nuisance. There is a good posting on how to avoid and remove these piggyback programs, click here.