Certificates - also known as digital IDs - allow for the guarantee of communications. What guarantee? Most modern email clients and many instant messenger programs have the capability to guarantee communication is private. With the ability to add digital signatures which keeps the email or conversation private these programs have built in tools to do just that. Users must install a certificate to provide the encryption needed to make sure the information can't be seen by unwanted individuals.
In order to use S/MIME (encryption and guarantee), home users or business owners must obtain and install a certificate into their applications to ensure privacy. In order to obtain a certificate, home users and business can request for free or purchase a certificate for their email account. These certificates are generally downloaded or requested from a public CA (certificate authority).
Personal certificates for home users (any individual) verifies the owner of the certificate by binding the owner to their email address. With businesses, the certificates are usually obtained through a certificate authority and are purchased by the company for a fee for a period of time.
Certificates basically ensure you know who the sender is, ensure encryption (privacy) and prevent tampering.