A React application comprises of various React components, the components are organized in a tree structure with a root component and other child / nested components. When the React application loads it creates the component tree in memory called the Virtual DOM. Whenever a component gets updated in the tree React runs an algorithm to determine what changes should be made to the DOM to reflect the changes in the UI, this process of comparing the changes to update the DOM is called Reconciliation.

React reconciliation is based on 2 assumptions