Both of these words describe substances that cause injury, illness, or death when they get into your body.
The difference is that poison gets into your body if you inhale it (breathe it in), ingest it (eat or drink), or touch it. Venom gets into your body if it is injected in, such as through a bite or a sting.
Certain snakes are venomous, because they bite you and inject their venom (although many native English speakers say “poisonous snake”). Scorpions and some spiders are also venomous, because they bite or sting you. There are plants and frogs that are poisonous – you will get sick if you touch them or eat them.
Sometimes people use poison to kill other people – by secretly putting it into their food or drink. When this happens, we say the victim was poisoned.