Main vocalist is basically the real deal, taking adlibs, choruses and generally harder parts in songs. Lead vocalist, however, takes a secondary role after the main vocalist, usually backing the main up, singing harmonies, handling verses or singing the lines when the main vocalist is busy screaming his or her head off for high notes. This is decided by the vocal prowess of the idols themselves, in terms of range, control and other musical aspects needed in a vocalist.
Main Vocalist - Gets a lot of parts too, but the harder parts. Usually the best singer of the group.
Lead Vocalist - Gets a lot of parts, but less challenging parts. Usually the second best singer after Main Vocalist. The Lead Vocalist is able to reach one or more higher or lower octaves than their fellow group mates.
So for T-ARA:
Soyeon is the main vocalist. (She is T-ARA’s melodysoyani for a reason)
Eunjung and Hyomin are the lead vocalists. (Eunjung is the high note queen, obviously. Hyomin’s voice is also no joke)
