Part of your hardware/software configuration can be traced by the websites you visit. For example a website can detect:
- Flash version.
- Web browser (IE, Chrome, Mozilla, etc.)
- Plugins in use on the browser
- Screen resolution
- Language
- Timezone
- OS (Windows XP, 7, 8, Linux, OSX, IOS, Android)
- Java version
- Fonts installed on your PC
Gathering all that information, a unique profile (a fingerprint) can be created and a history of the behavior of the profile stored (have this profile clicked ads in the past? which kind of ads? how long this profile stays?, etc...)
Next time that unique profile visits the website the system can determine which ads should be shown to him.