The extended SMTP functions are identified by Extended. For example, the identification of the client with ESMTP is done by the Extended Helo (EHLO) command instead of HELO, as in classic SMTP protocol.
Important extensions are the use of 8-bit characters in the ASCII character set, instead of the 7-bit characters in SMTP, and the size limit of an e-mail that the server accepts. With ESMTP, the same e-mail can be sent to multiple recipients at the same time.
The extensions are described in RFCs 1426 and 1427, but since the current SMTP versions contain all the features of ESMTP, ESMTP no longer has any significance.