What is the difference between a constituent and a voter?

A constituent is any person within a legislative district, whether they are registered to vote or not. Even if they are not of voting age.

A voter is someone qualified (must be a citizen & legally of age to vote. Must live in the district) and registered to vote.

For example: a 15 year old is a constituent but not a voter. A 20 year old registered to vote is a possible voter and is also a constituent.