"sendmail" is a relatively common term. It means different things in different contexts.
"sendmail" is the name of package for sending, routing and delivering SMTP-based e-mail between systems. See: www.sendmail.org
"sendmail" also happens to be the name of a utility to send e-mail messages from the command-line. Using an external utility (as opposed to the PHP built-in mail functions) give you more flexibility in how your message is put together. For example, the ability to specify the "From" address, multiple CC/BC/etc. addresses, multiple attachments, and mail routing on a message-by-message basis.
-- Michael
Darkstreak Computing & Innovations
www.darkstreak.com