Did a whole presentation on this.
Physical - when a guardian physically harms a child. (Shaken baby syndrome can go under this category as the parent physically shakes the baby to death)
Emotional/mental- When a guardian mentally and emotionally abuses the child with words (namecalling, etc) , mind games, manipulation, neglecting, etc .
Sexual - when a guardian sexually abuses a child.
But really there is lots of opinion/controversy on what classifies as child abuse, but ultimately, if a parent inflicts severe emotional/physical injury to the child, it is child abuse. Where a child is neglected or injured and does not feel genuine love and support from the parent.