Cohesion
When like molecules are attracted to each other.
Example:
Water molecules are attracted to each other through hydrogen bonding and create a phenomenon known as surface tension -- allowing insects to walk across the surface of water.
Adhesion
When molecules are attracted to other molecules that aren't the same type.
Example:
When water clings to a leaf, the water molecules are attracted to the molecules in the leaf. The water molecules form a droplet due to cohesion forces that attract them to each other.
