Spyware is a type of malware which gets installed in our computer, mobiles etc. and steals information from our machine without our permissions. Sometimes it can be very harmful for us, as it can leak some of our secret information from our machine. It is used by many companies to gather the internet browsing details from our system and flood our system with worthless advertisements and fake security threats There are four types of spyware like system monitors, trojans, adware, and tracking cookies.
To know more about it :- http://en.wikipedia.org/wiki/Spywares