MTA is an acronym for Mail Transport Agent. It is the software that works behind the scenes to transport E-Mail messages from one computer to another. MUAs (such as mutt, thunderbird, sylpheed, evolution, kmail) hand off newly sent messages to an MTA. MTAs talk to other MTAs, and either deliver mail locally or hand it off for delivery to an MDA/LDA if it was destined to the local system.
MTA is a generic term and usually refers to one of these popular software packages:
Postfix is Wietse Venema's secure, fast and flexible mailer. Default on SUSE Linux and NetBSD.
Exim is Philip Hazel's flexible mailer. Default on Debian GNU/Linux.
Sendmail the original BSD mailer. Default on FreeBSD; a Sun spinoff is used on Solaris.
Courier was inspired by qmail, but intends to do things right.
qmail is an obsolete and unmaintained server. Its POP3 part can be taken over by Dovecot. Qmail started off boasting about speed and security in the mid-1990s, but has lots of unfixed bugs (this document includes patches where known), among them security bugs that remain unfixed, and the security guarantee (500 USD) denied. If you really intend to continue using it, read Dave Sill's Life with qmail which contains instructions to work around some of qmail's security issues.
Some people also subsume mail fetching utilities under the MTA category, among them:
fetchmail a fast mail retriever for the POP2, POP3/KPOP/SDPS, IMAP2/IMAP4, ODMR and ETRN protocols, SSL and Kerberos capable. It forwards the retrieved messages to SMTP/ESMTP, LMTP servers or into an LDA. Developed out of Carl Harris's popclient by Eric S. Raymond. Fetchmail is now maintained by MatthiasAndree and Rob F. Funk.
getmail a POP3/SDPS and IMAP4 (with SSL) enabled mail retrieval utility written in Python. Developed by Chales Cazabon.
These mail fetching utilities can be used to store mail for later retrieval by dovecot.
Contrast this to the message delivery agent (MDA), sometimes called local delivery agent (LDA).