Portable Network Graphics is a ubiquitous file format for conveying still images. It is used on the web and in various document systems, and it has a decent level of lossless compression.
A PNG file is composed of an 8-byte signature header, followed by any number of chunks that contain control data / metadata / image data. Each chunk contains three standard fields – 4-byte length, 4-byte type code, 4-byte CRC – and various internal fields that depend on the chunk type.
The PNG file format starts off with a magic signature, and is followed by any number of chunks all with a uniform syntax. This design is similar to other popular multimedia file formats, like: BMP, TIFF, WAV, AVI, general RIFF. It’s different from plain text file formats (examples like Netpbm surprisingly exist), XML’s hierarchical elements, a ZIP container of subfiles, PDF’s custom binary format, etc.