Its Illegall to spy someone..
Well Spyware is just normal software being used in the wrong way.
The definition Spyware is some software the enters in your computer from a website trough your browser and collects personnal info about you using cookies or Executables (via trojan or atached to normal programs as worms)
Its used by companies (they study your habits and redirect your browser so you buy their stuff) and by guys that want to make money from uproteted persons (for ex. stealing credit cards numbers and codes).
The best thing you can do, is buy a good Internet Security (like Kaspersky Internet Security or Panda Internet Security or Norton internet Security) that protects you from almost all invasions (like spyware,adware,malware,virus,trojan,rootkits,bots,etc.. from the internet)
hope that helps