If you previously got read my post about what is SSL Cert, then I believe you some how already what is a SSL cert and what it is for.
Unlike other SSL certs issuers, CAcert do not insured you with any amount of money if your website is hacked by any jack ass. CACert is just running as a CA root, where it will act like other CA roots, to validate and identify your website’s certificate. But they do it for free and they will not insured you with anything.
The only problem with CAcert now, is their PKI Keys are not installed in any web browsers by default. Others like Verisign and Geotrust etc, all have their PKI keys installed by default in most of the web browsers available nowadays.
So in order for your web browser to be able to identify SSL certs issued by CAcert, you may go to http://www.cacert.org/index.php?id=3, click the Root Certificate (PEM Format), and the Intermediate Certificate (PEM Format) to install the PKI keys for your web browser.
Then, whenever you go to those websites which are using SSL certs issued by CAcert, you will found that it will not saying that the site’s SSL cert are unable to be identified, instead, it will directly show you the website and when you click on the icon at the address bar, it will be stated that the cert is issued by CAcert.