• the browser version
• the timezone and preferred language
• the set of video or audio codecs that are available on the system
• the fonts installed on the system
• the state of the browser's settings
• the computer's display size and resolution

A website can retrieve information like this by executing JavaScript and CSS on the device, and by combining this data can often create a unique fingerprint for a browser, which can then be used to track users across the web.

Web standards are designed in such a way as to minimize the ability of a website to collect identifying information, and browsers typically add their own protections as well.