Final answer:
useEffect is a React hook for handling side effects after renders, while useCallback is for memoizing callback functions to prevent unnecessary renders. Both are used in React to manage component lifecycle and rendering efficiency.
Step-by-step explanation:
The question refers to two hooks in the React.js library, useEffect and useCallback, which are used to handle side effects and memoize callbacks, respectively.
useEffect is a hook that tells React to run some code based on dependency changes between render cycles. It's commonly used to interact with an API, timers, or manually DOM manipulations after the component renders. The function passed to useEffect will run after the render is committed to the screen, and you can optionally specify an array of dependencies.
useCallback, on the other hand, is used to memoize callback functions. It returns a memoized version of the callback that only changes if one of the dependencies has changed. This is useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders.
Both hooks are essential for optimizing React applications and managing side-effects or avoiding unnecessary re-renders of components.