What distinguishes an ID from a class in CSS?

In CSS, an ID is characterized by its uniqueness; it is specifically intended to be associated with a single element within a document. This means that whenever you define an ID for an HTML element, that ID should not be assigned to any other element in the same HTML document. This uniqueness allows for greater specificity in targeting and styling that particular element with CSS or manipulating it with JavaScript.

On the other hand, classes are meant to be reusable and can be applied to multiple elements, which facilitates consistent styling across different components of a web page. While you can have both IDs and classes for an element, the fact that IDs are unique differentiates them from classes, which can be shared by many elements.

This fundamental distinction is crucial for developers to understand, as it impacts how styles are applied, how elements are accessed through JavaScript, and how browsers render the structure of a web page. Additionally, the use of IDs for element targeting often comes into play when considering accessibility and best practices in web development.