A textarea is an HTML element that allows users to input multi-line text. It is commonly used in web forms for comments, messages, or any other type of text entry that requires more space than a single line. The textarea can be resized and customized in terms of width and height, making it versatile for various applications. In web development, the textarea is defined using the `<textarea>` tag. It can include attributes such as `rows` and `cols` to specify its size, and it can also be styled with CSS for better appearance. This element is essential for enhancing user interaction on websites and applications.