Adware, spyware and computer virus share some similarities, one of which is that all three are major nuisances for computer users. Let’s differentiate the three.
Spyware is software that does not intentionally harm your computer. What they do is that they create pathways wherein someone else aside from the computer owner can communicate with the computer. Normally spywares record the various types of web sites you visit which are later used by web advertisers to allow them to send you unwanted emails and pop-ups.
This is why spyware are usually frowned upon and greatly avoided. They are more intrusive than adware. Spyware have their own separate executable programs which allow them to record your keystrokes, scan files on your hard disks and look at other applications that you use including but not limited to chat programs, cookies and Web browser settings.