text: Generally speaking, adware is software that gets installed on your machine, usually *with* your consent and installation of another legitimate program, that will display ads while that software is being used.  The Spyware term on the other hand is meant to classify software that is unknowingly installed on your machine, and it tracks your activity, and even reports it or sells it to others without your knowledge.  The Wikipedia site has brief but very good writeups on spyware, adware and malware.