What is the main difference between an "id" and a "class" in HTML?

The main difference between an "id" and a "class" in HTML is based on their uniqueness and how they are used within a document. An "id" is a unique identifier that must be assigned to a single element within a page, meaning no two elements can share the same id value. This makes it particularly useful when you need to target a specific element for styling with CSS or for manipulation with JavaScript.

In contrast, a "class" is a reusable identifier that can be assigned to multiple elements. This allows for applying the same styles to many elements at once, which facilitates consistent design and reduces redundancy in the code.

The correct answer emphasizes this differentiation: the id is unique to a single element, while classes can be applied to multiple elements throughout the HTML document, enabling greater flexibility in styling and scripting.