Shliach means messenger. Some Orthodox groups send young couples out to revive a Jewish community. They are often called shliach.
A Chazan (cantor) what is called a Shliach Tzibur (a messenger of the congregation) leads the service. In orthodox synagogues members of the congregation typically rotate and take turns leading the service.See also hazzan
The person leading the service, is usually called a shaliach (or shlicha if female), which is short for shaliach tsibor (שליח ציבור), "public representative".the Shaliach in many synagogues is either a Rabbi or a Cantor, but this is not a requirement to be a shaliach, and many synagogues don't have either one.To lead a Jewish service, a person only has to be Jewish and over the age of 13 (and a few other situational requirements). Orthodox synagogues usually (but not always) require the shaliach to be male unless it is an all-female group.