SASL stands for "Simple Authentication and Security Layer". SASL itself is nothing more than a list of requirements for authentication mechanisms (see [wiki:Authentication Authentication]) and protocols to be SASL-compatible. IMAP, POP3 and SMTP protocols all have support for SASL.
Many people confuse SASL with one specific SASL implementation - Cyrus SASL library. Dovecot has its own SASL implementation which may at some point be separated from Dovecot itself to "compete" against Cyrus SASL library in server side.