PSD is the proprietary format of Adobe's graphic design software, Photoshop. PSD files can be saved in either RGB or CMYK mode, and they also allow for customizing the number of colors and storing them. Additionally, PSD files can preserve Photoshop layers, channels, paths, and other information. It is currently the only format that supports all image color modes, though it tends to have a large file size. PSD files are widely compatible within many graphic design programs (such as CorelDRAW, Illustrator, After Effects, etc.), and they can also be used in some other types of editing software, such as the Office suite.
Below, we will introduce a few excellent websites where you can download PSD files: