Google Web Fonts identifies browsers using User Agent Sniffing technique. Check out the following examples.
If it's Google Chrome, a WOFF font is returned:
$ curl -A 'Mozilla/5.0 (Windows NT 6.2; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/32.0.1667.0 Safari/537.36' http://fonts.googleapis.com/css?family=Open+Sans:400,700
@font-face {
font-family: 'Open Sans';
font-style: normal;
font-weight: 400;
src: local('Open Sans'), local('OpenSans'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/cJZKeOuBrn4kERxqtaUH3T8E0i7KZn-EPnyo3HZu7kw.woff) format('woff');
}
@font-face {
font-family: 'Open Sans';
font-style: normal;
font-weight: 700;
src: local('Open Sans Bold'), local('OpenSans-Bold'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/k3k702ZOKiLJc3WVjuplzHhCUOGz7vYGh680lGh-uXM.woff) format('woff');
}
If it's Internet Explorer 10, an EOT and a WOFF font are returned:
$ curl -A 'Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)' http://fonts.googleapis.com/css?family=Open+Sans:400,700
@font-face {
font-family: 'Open Sans';
font-style: normal;
font-weight: 400;
src: url(http://themes.googleusercontent.com/static/fonts/opensans/v6/cJZKeOuBrn4kERxqtaUH3fY6323mHUZFJMgTvxaG2iE.eot);
src: local('Open Sans'), local('OpenSans'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/cJZKeOuBrn4kERxqtaUH3fY6323mHUZFJMgTvxaG2iE.eot) format('embedded-opentype'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/cJZKeOuBrn4kERxqtaUH3T8E0i7KZn-EPnyo3HZu7kw.woff) format('woff');
}
@font-face {
font-family: 'Open Sans';
font-style: normal;
font-weight: 700;
src: url(http://themes.googleusercontent.com/static/fonts/opensans/v6/k3k702ZOKiLJc3WVjuplzHZ2MAKAc2x4R1uOSeegc5U.eot);
src: local('Open Sans Bold'), local('OpenSans-Bold'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/k3k702ZOKiLJc3WVjuplzHZ2MAKAc2x4R1uOSeegc5U.eot) format('embedded-opentype'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/k3k702ZOKiLJc3WVjuplzHhCUOGz7vYGh680lGh-uXM.woff) format('woff');
}
In response to your second query: Web Fonts aren't permanently installed on the client side. They reside in the browser cache temporarily. Clearing the cache prompts the font to be downloaded again. If two websites serve the same font from different URLs, duplication occurs.
However, if the client's operating system has the font installed at a system level, it uses that version without downloading. This is where the local sources in the CSS come into play.