What distinguishes an ID selector from a class selector in HTML?

An ID selector is characterized by its uniqueness within a given HTML document. It can only be applied once on a page because each ID must be unique to identify a single element. This means that if you assign an ID to an HTML element, no other element can have the same ID, which enforces that distinct identification.

In contrast, class selectors can be used multiple times within the same document. This allows for flexibility in applying the same styling or behavior to several different elements without the risk of duplication, as class names are not required to be unique.

The uniqueness of ID selectors is particularly useful for targeting single elements for specific styling or JavaScript manipulation. It's a foundational concept in HTML and CSS that helps maintain clear document structure and effective styling strategies.