In some ways all abuse is emotional abuse. The very fact of being abused in any way causes emotional damage.
Emotional abuse specifically refers to abuse that directly plays on the emotions. Expecting a young child to have the same emotional maturity as an adult and burdening them with things they shouldn't know. Constantly telling someone they're worthless or sending extremely mixed messages, ridicule, jokes which damage the self-image or self-esteem of the person in question. Yelling at someone can be emotional abuse.
There is a certain degree of direct overlap between MentalAbuse and Emotional abuse. They are both very damaging, and leave no visible marks. Sometimes you can tell the difference between mental and emotional abuse when you separate out the act and the reactions: making unreasonable demands on someone is probably more mental abuse, while attempting to elicit guilt to get them to comply would be emotional abuse. However both can happen at the same time (as in "After everything I've done for you, you've never bought me flowers! If you were really romantic, you would buy me flowers on our anniversary!" -- the demand is mental abuse, the guilt and pressure are emotional abuse).