In Bootstrap, ".container" and ".container-fluid" are used to create responsive layouts, but they differ in how they handle width. ".container" provides a responsive fixed-width container, while ".container-fluid" provides a full-width container that spans the entire width of the viewport.

<!-- BEGIN COPY / PASTE -->
        <div class="container">
            <!-- Content inside a fixed-width container -->
        </div>

        <div class="container-fluid">
            <!-- Content inside a full-width container -->
        </div>
        <!-- END COPY / PASTE -->

1. Use ".container" when you want your content to have a maximum width that adjusts at different breakpoints.
2. Use ".container-fluid" when you want your content to stretch across the entire width of the viewport, regardless of the screen size.
3. Both classes are responsive, but they serve different layout needs depending on the design requirements.