How they do that?

Code question, but I dunno how they did it, so...

I just had a html email that contained a "contact us" link. I clicked it, and was taken to the "Compose reply" page of the webmail service I was using.

How? How did they know the url of the page? How did they get it into the email body? Sorry if this is thick, but I'm confused.

an URL would be good. so they signed you in and all, eh?

It's done by the mail service rather than the sender, the sender will use a simple Mailto:

Most web-based email services use query string parameters so they pass data on their links.

If you use Yahoo, look at your address book. They set each email address as a link with a URL something like:

http://xyz.mail.yahoo.com/ym/Compose?&To=somebody@somewhere.com

So if you're sending an email to someone with an @yahoo.com address, you know you can embed a link like the above and just change the email address.