CSS units are used to define the dimensions of elements in a web page. These units specify how wide, tall, thick, or spaced an element should be. CSS provides various size units to accommodate different design needs and responsiveness requirements. The most commonly used CSS units include px (pixels), em, rem, %, vw, vh, vmin, vmax, ch, ex, and %. The best unit for sizing CSS depends on the specific design requirements. For responsive design and scalability, relative units like em, rem, and percentages are often preferred. However, for fixed layouts or when pixel precision is necessary, px (pixels) may be suitable. The choice ultimately depends on the design goals and the nature of the layout.