How are sessile leaves different from ordinary leaves?

The main difference between sessile and ordinary leave is petiole.

Sessile leaves lack sessile while ordinary leaves have a petiole. In botany, sessility (meaning "sitting", used in the sense of "resting on the surface") is a characteristic of plant parts that have no stalk. Flowers or leaves are attached directly from the stem or peduncle, and thus Sessile leaves lack a lack a petiole or pedicel. The leaves of the vast majority of monocotyledons lack petioles.