A UI kit is a collection of assets and their style attributes that designers can use to build the user interface of a digital product and can be static or coded. In contrast to style guides that provide recommendations and best practices on how elements could be designed, UI kits demonstrate their behavior. UI kits are more focused on illustrating all possible states of an element, including the error state.

Some companies create internal UI kits that their designers can use exclusively. Others turn to pre-made external UI kits that they adapt for their own use. External UI kits are usually used in the early stages of product creation when resources are limited.