Although the thought of software taking a piggyback ride on another sounds funny, the results of downloading and using such software are not.
Piggybacking occurs in two forms, malicious and non-malicious. The non-malicious kind comes along with “decent” software. For example, reputed software providers like Adobe include an already pre-selected option to install a web toolbar within one of their product installations. If you are not closely watching the install process and blindly hitting “next”, the toolbar will be installed. While this may annoy you later, the toolbar can be uninstalled and to be fair, you did have the option of not installing it. The main thing here is that the intent of the company is not malicious in nature, but is geared towards self or product promotion.
When it comes to the malicious kind of piggybacking, you will not be made aware of its presence. Most often, it comes through free games or tools that you find from unknown vendors. Once inside your system they can do whatever task they were designed to like throwing out pop-up windows, redirecting websites, steal your information, etc. In short, it can cause havoc and that is something you do not want.
You can avoid this type of software by not running pirated software, downloading from disreputable sites, avoiding music sharing programs, etc. Even if you do not do these things, running a virus scanner and a spyware scanner regularly on your computer will help you. But remember, prevention is the best cure.