Spyware is a type of software that work in the backend, often silently, performing its objective without the knowledge of its user.
1. Marketing Intent
On marketing perspective, spyware is installed into visitors PCs and used by its owner to send marketing or promotions news to the end user browser.
It's not clear what other information that is being collected and sent out by this spyware but a wild guess would be sites visited, time spend or maybe what type of search information the user sends out. Age group, location and language used could also be useful for web marketing analysis.
One top name, AVG anti virus is known to bundle their toolbar and homepage in some highly sort after freeware like Miro and PeaZip during installation.
If you're not careful during installation, and click 'next' all the way to accept default settings, you'll ended up installing AVG as your homepage. It's has pretty nasty behaviour, because once you installed AVG doesn't allow you to set other site as your homepage.
You will need in uninstalled it from your control panel to remove it.
Mybrowserbar is another toolbar with this nasty behaviour that is bundle with free pdfcreator from the sourceforge.
On a more stricter perspective, this type of spyware is also known as Adware (Sneaky advertisement sent right to your computer screen).
Some installer are intentionally made in sure a way to confuse user, giving them the thought that they have to installed the bundle spyware as part of the agreement to use the freeware. One example I encountered is the Babylon toolbar, you can take a look and judge for yourself.
It's possible there are many other who are also using this unscrupulous strategy to expand their marketing reached on the internet but one thing you can do is, beware of freeware installation nowadays. If you happen to find any brand name using this dirty tactics - just boycott them!
2. Malicious Intent
Spyware client is installed to unsuspecting victim. Keyloggers is one commonly used type of spyware for hacker to steal information. Beside credit card information, that what most people can think of, gaming account, social media account, and even login information to website are also valuable.
Diablo III, is a hit game for gamers. Their gaming account certainly do carry a value, specially top players.
Social Media account, is also valuable. Your friends' email account can be valuable to someone preying on social media site.
3. Monitoring Intent
Some network monitoring software, do comes with client that can be deployed to all its client workstations in a network as monitoring agent.
Client agent installed at individual workstation can be then set to either collect information about the activities of a user, control access to network resources, or simply disallowed access to a network resource.