From:
http://www.computerhope.com/jargon/a/adware.htmAlso known as malware, sneakware, or spyware, adware is a type of software program thats installed without a users consent or knowledge while another program is being installed on the computer. Much like spyware, an adware program is used to track an individuals Internet activities and/or habits to help companies advertise more efficiently.
Adware is commonly installed on a computer with free programs to help support the development of that program. Like spyware, adware can be located and removed from a computer using software utilities available on the Internet. See our spyware page for examples and recommended programs.
Additional information about Spyware, Malware and Adware programs being installed and/or your browser being hijacked can be found on
document CH000578.
As far as Incredimail being spyware / adware check out:
http://email.about.com/cs/incredimailtips/qt/et063003.htm