What is the difference between a stick insect and a leaf insect?

Stick insects belong to the family of Phasmatidae, while leaf insects belong to Phylliidae.

Both are from the order of Phasmatodea, and are therefore easily confused. For example, Extatosoma Tiaratum is commonly known by the name 'Spiny Leaf Stick Insect'. Having 'Leaf' in the name can confuse it with the family Phylliidae, though it's actually a Phasmid. For this reason, when researching, and writing reports, it's important to be certain of which family an insect belongs to.

If you want a simple answer, stick insects tend to look more like sticks, and leaf insects like leaves, though again, they can be easily confused.