Fluid layouts are web design structures that adapt to the size of the user's screen. Unlike fixed layouts, which have set widths, fluid layouts use percentage-based widths, allowing elements to resize proportionally. This flexibility ensures that content remains accessible and visually appealing on various devices, from smartphones to large desktop monitors. By utilizing fluid layouts, designers can create a seamless user experience. This approach is particularly important in today's world, where users access websites on a multitude of devices. Tools like CSS and frameworks such as Bootstrap help developers implement fluid layouts effectively, enhancing responsiveness and usability.