Neglect is as serious as directed abuse. Being denied love and attention, being left to fend for yourself, is an instinctual death sentence where instinct and ancestral memory remind us that we require tribe or clan support for our survival.
There are many ways that neglect can happen to a child, and as many ways to be neglected as there are to be abused. Any needs that are ignored, played-down, pushed away, abandoned, unaddressed, dismissed, etc. become neglected needs. And children have many varied needs.
One can be sexually neglected if parents don't properly give you a sexual education, or don't touch you in appropriate ways (hugs, huggles, asexual caresses outside of errogenous zones). Touching appropriately helps a child learn appropriate boundaries for non-sexual contact, while still showing them that they are touchable and physically comfortable/likable people.
You can be physically neglected if you aren't given proper hugs, exercise, food, etc.
You can be spiritually &/or religiously neglected if parents don't give you proper spiritual leadings or allow you to collect information about spirituality/religion.
You can be emotionally neglected, if your emotions are dismissed, or your caregivers do not recognize or acknowledge your emotions.
You can be spiritually neglected, if you are not given anything more than the physical to believe in, if you are not acknowledged when you have a spiritual experience, if your experience is dismissed or goes unacknowledged.
Neglect may affect a child's self-image, self-esteem, emotional health, trust, etc. Sometimes a child feels that negative attention -- i.e. abuse -- is better than the lonliness that can come with neglect.
Neglect can also arouse very primative fears in the face of infant or early childhood helplessness, causing a child to panic about things like their health, their food, the dangers in the environment, etc. Even if food comes well before starvation, if a child suffers hunger, they start to doubt that food will come when they need it to…
Neglect can also be caused by one or both parents (and other persons) not intervening between the child and other abuser(s) in the child's life: since children generally regard adults as being omniscient, this lack of intervention on the child's behalf could seem to be consciously allowing the abuse to happen and continue. Someone who doesn't know the abuse is happening, and honestly does not see signs of abuse, can be seen by the child to be in cahoots with the perpetrator(s), and this can cause the child yet more damage, even though that person technically didn't do anything wrong.