Certificates are a Public Key and Identifying Information
Within public key cryptography (such as that used in SSL and TLS), you have both a private key (that you keep secret), and a public key (that you share widely).
In order to avoid MITM (Man In The Middle) attacks, rather than publishing just the raw public key, you normally share a Certificate. The Certificate contains your public key, along with information identifying you (such as the hostname of your website, and your organisation). The identifying information is authenticated by a Certificate Authority (CA), and can be used to ensure you're talking to the right person.
Certificates are normally issued by a Public Certificate Authority, but they can be self signed (the certificate is its own CA), or issued by a private CA.
