In JavaScript, an array is a data structure that stores a collection of values or objects of any data type. The elements of an array are accessed by their index number, which starts from 0.

For example, an array of numbers can be defined as:

let numbers = [1, 2, 3, 4, 5];

A set, on the other hand, is a built-in JavaScript data structure that allows you to store unique values of any data type. A set does not store duplicate values, so if you try to add an element that already exists, it will not be added.

For example, a set of numbers can be defined as:

let numbers = new Set([1, 2, 3, 4, 5]);

The main difference between an array and a set is that arrays allow duplicates, while sets only store unique values. Another difference is that arrays have a fixed length, whereas sets can dynamically grow or shrink in size.

In terms of operations, arrays have more built-in methods and are commonly used for iterating, sorting, and filtering data. Sets, on the other hand, have methods for adding and removing elements and are useful for checking whether a value exists in the set or not.

Overall, arrays and sets serve different purposes in JavaScript, and the choice of which one to use depends on the specific use case and data requirements of the program.