What's the difference between an "attribute" and a "property"?

Attributes are defined on the HTML markup but properties are defined on the DOM. To illustrate the difference, imagine we have this text field in our HTML: <input type="text" value="Hello">.

const input = document.querySelector('input');
console.log(input.getAttribute('value')); // Hello
console.log(input.value); // Hello

But after you change the value of the text field by adding "World!" to it, this becomes:

console.log(input.getAttribute('value')); // Hello
console.log(input.value); // Hello World!