Heroin is not technically categorized either as a depressant or stimulant, but rather as an opiate. It is a highly addictive drug derived from morphine, which is obtained from the opium poppy plant.
It sounds like what you may be asking about is how the drug affects your body and it's overall functioning. Heroin is a highly addictive substance that has a number devastating affects among its users. Click on our answer to Heroin? for more information on the affects of heroin or log on to National Institute on Drug Abuse.
If you are considering using heroin - please, please do not - it is not worth risking your life for something so dangerous. If you are already using or know someone who is, you need to get help. Reach out to a parent teacher or other trusted adult who can provide support.