PNG (pronounced "ping"; Portable Network Graphics) is a lossless bitmapped image format. PNG was created to improve upon and replace GIF (Graphics Interchange Format) as an image-file format not requiring a patent license. PNG was developed to replace GIF (gif) since GIF contained a proprietary compression (LZW). GIF also had other drawbacks such as limited colour range. The compression used for PNG does not compress photographs much and is hence not useful for complex images with colour gradients. Compression is, however, very good for line graphics.
In (La)TeX, questions concern inclusion of graphics into documents and issues where the compression may become an issue.