They certainly seem similar to plants on the surface. It turns out that they are not plants.
- They don't have chloroplasts, the important organelle where photosynthesis happens.
- They don't make their own food like plants do. Instead, they absorb nutrients from their surroundings using digestive enzymes.
- They are more closely related to animals than plants.
I never would've guessed that animals and fungi are relatives!
Further Reading: Three Reasons Fungi Are Not Plants