To begin using Masonry.js for building Pinterest-style responsive grid layouts, you first need to include the library in your project and set up a basic grid structure. You can add Masonry.js either by linking to a Content Delivery Network (CDN) or by downloading the file and referencing it locally. The CDN approach is faster for prototyping, while the local method is better if you want to work offline or bundle the script with your project files.

A grid container is the parent element that holds all your grid items. Each grid item is a direct child of this container and represents an individual block in your layout, such as a card or image box. Masonry.js works by dynamically positioning these grid items so that they fit together tightly, minimizing gaps and creating a visually appealing, responsive arrangement.

Next, you'll see how to include Masonry.js in a simple project, create a grid container, add a few items, and initialize the Masonry layout with JavaScript.