SSL certificates are an essential security measure. There are several different types of SSL certificates available. One is a dedicated SSL certificate. This type of security certificate is useful for one domain as the name suggests. The SSL certificate, by way of technical explanation, is also referred to as secure socket layer. The protocol makes identification of a web-source and subsequent (secured) data exchange possible. Data is transmitted by way of encryption; and once reaching its destination is (subsequently) decrypted. That is the best way to put it, without creating a great deal of technical confusion. The certificate providing specifications of 128 bit encryption is the average. You can purchase certificates offering more protection; or less, and everything is relative with respect to (associated) cost(s).
The other form of SSL certificate is referred to as shared. The shared SSL certificate is just as the name suggests: an SSL certificate shared by more than one site. Accordingly, it is not as expensive as the dedicated SSL certificate.
The wildcard SSL certificate is intended for websites with sub domains incorporated onto the site. In this way, not only is transmission of data protected on the main site; data exchanged is, as well, protected with regard to the site's sub domains.