Exploitation is an act where a person is treating another person unfairly or in a way that is wrong, in order to receive personal gain. Examples of exploitative behavior include acts where a person is offering money to someone who cannot refuse in return for personal benefits or uses manipulation or threats for such gain.
Children are often more vulnerable to exploitation as some people will take advantage of their young age, their lack of ability to refuse or their misunderstanding of a situation, for their own profit. An example of Child Exploitation for instance is forcing a young child to work in physical labor. This is a situation that harms the child and the child has no ability to refuse while the employer is gaining profit from their work.
When the personal gain that is received as a result of the exploitation is sexual in manner, it will be Child Sexual Exploitation, for instance in the cases of children exploited in prostitution.
And just as the name suggests, if the act of sexual exploitation happens online, or with the use of online technology, it will be considered Online Child Sexual Exploitation (OCSE).