What does ‘background-size’ do in CSS?

Some ways of using an image in a background.

.stretch{
/* Will stretch to specified width/height */
background-size: 200px 150px;
}
.stretch-content{
/* Will stretch to width/height of element */
background-size: 100% 100%;
}

.resize-width{
/* width: 150px, height: auto to retain aspect ratio */
background-size: 150px Auto;
}
.resize-height{
/* height: 150px, width: auto to retain aspect ratio */
background-size: Auto 150px;
}
.resize-fill-and-clip{
/* Resize to fill and retain aspect ratio.
Will cause clipping if aspect ratio of box is different from image. */
background-size: cover;
}
.resize-best-fit{
/* Resize to best fit and retain aspect ratio.
Will cause gap if aspect ratio of box is different from image. */
background-size: contain;
}