There are viruses, there is spyware/adware/malware, and then there are cookies. All are different in purpose. In some cases, they pose a potential damage to your computer.
A virus intends to do damage to your computer; therefore, it poses the most serious threat. That is why installing an anti-virus program on your computer is essential. 
Depending upon which of the thousands of malicious software has infected your system, it may want to do any of the following:
(1) Take information such as your contact list or browsing history and use that for some nefarious purpose.
(2) Redirect your browser to paid-per-click unintended, irrelevant, and/or offensive sites.
(3) Bombard you with paid-per-view popup. While these do not pose a serious threat, they can be very debilitating. Many times, their removal process exceeds the average user’s abilities.