A SSL certificate will secure your site using encryption and will validate your identity on the Internet. This is especially useful if you handle confidential information (credit cards) or if your customers use a password to access your site.
If your business requires a stronger level of trust, you can chose to use an extended validation certificate. Using such a certificate, a green bar containing the name of you company will appear in the browser's navigation bar.
(1) : If you host your website with us, SSL certificates are served through SNI. This will work in all modern browsers. The most notable exception is Internet Explorer under Windows XP (which is not very modern). To be fully backward compatible, you should consider getting a dedicated IP for your hosting account.
(2) : If www.domain.com is ordered, the certificate will protect both www.domain.com and domain.com