Network information
The server sees the IP address your request came from, which indicates an approximate location and network provider.
Browser details
Requests include the browser type, language preferences and screen-related hints used to display the page correctly.
Referrer
Browsers may tell the site which page linked to it. Many browsers now trim this to the origin only.
What it does not see
A page cannot read your files or other tabs. A Vortex Market information page that asks for more than it needs, such as logins, is overstepping.