One of the several new features in Chrome is the addition of HTTP Strict Transport Security. HSTS allows a site to request that it always be contacted over HTTPS. HSTS is supported in Google Chrome, Firefox 4, and the popular NoScript Firefox extension.
The issue that HSTS addresses is that users tend to type http:// at best, and omit the scheme entirely most of the time. In the latter case, browsers will insert http:// for them.
However, HTTP is insecure. An attacker can grab that connection, manipulate it and only the most eagle eyed users might notice that it redirected to https://www.bank0famerica.com or some such. From then on, the user is under the control of the attacker, who can intercept passwords etc at will.
An HSTS enabled server can include the following header in an HTTPS reply:
Strict-Transport-Security: max-age=16070400; includeSubDomains
When the browser sees this, it will remember, for the given number of seconds, that the current domain should only be contacted over HTTPS. In the future, if the user types http:// or omits the scheme, HTTPS is the default. In fact, all requests for URLs in the current domain will be redirected to HTTPS. (So you have to make sure that you can serve them all!).
For more details, see the specification.
Preloaded HSTS sites
There is still a window where a user who has a fresh install, or who wipes out their local state, is vulnerable. Because of that, Chrome and Firefox share a "Preloaded HSTS" list. These domains will be configured for HSTS out of the box.