Steric effects are non-bonding molecular interactions that influence both the shape and reactivity of ions and molecules. Steric effects result from repulsive forces between functional groups arise as a result of overlapping electron clouds. Steric effects within a molecule can dictate the shape (or conformation) of the molecule and influence the properties of that molecule and how it reacts.
Steric hindrance is the consequence of steric effects on the rate of a reaction. Steric hindrance is used to refer to the slowing of a chemical reaction rate due to steric bulk interactions. Steric hindrance usually refers to the interaction of intermolecular groups.
Understanding steric hindrance can allow the design of chemical reactions to force regioselectivity or stereoselectivity in a reaction or to minimize undesirable side reactions.