To my mind, a singer sings. Even if they've got a terrible singing voice, even if they're trying but failing, if the intention is to set words to a tune, they're singing. Even if those words are "Shana-na-na, sha-na-na-na-na, yip-yip-yip-yip-yip-yip-yip-yip bmm-bmm-bmm-bmm-bmm-bmm, get a job", they're still words, to a tune, ergo singing.
The moment the tune aspect entirely and intentionally disappears from the performance, e.g. rap etc., it's vocalising by a vocalist, not a singer. Michael Bublé and Tom Araya are singers, Chuck D and Dani Filth are not.
Yes this is a bit of an over-simplification and I'm sure there are gaping holes that can be picked in it, but I know what I'm trying to say.