From LQWiki

Services are provided by a wide variety of server software. Like the name suggest, a server software provide one or more services to requesting clients. The services provided can range from information delivery to execution of a program to control a robot, local or remote.

Among the more common services provided by Linux powered computers are:

• Web pages - Apache
• Mail delivery - Sendmail, qmail
• File Transfer (ftp) - proftpd
• Cache proxy - Squid
• Database - MySQL, PostgreSQL

Services are also the Windows equivalent to Linux/Unix daemons.