Everytime you access a website a server is serving you files. Apache (and most web servers) keep logs of this. With Apache defaults you get IP address, the route accessed, and the User-Agent of the user. This is rudimentary information, but if you have these logs from multiple sites, it's pretty easy to roughly track someone. Tracking images in emails use this same principle, a unique link to krick.png is put in an email sent to you, and if it gets served by the server (shows up in the access logs) it's pretty reasonable to assume that you read the email.

If you want to see a simplified version of what this log looks like, run 'python -m SimpleHTTPServer' and visit localhost:8000.