JPG
JPG, also known as JPEG, is a file format that can contain image with 10:1 to 20:1 lossy image compression technique. With the compression technique it can reduce the image size without losing the image quality. So it is widely used in web publishing to reduce the image size maintaining the image quality.
PNG
PNG or Portable Network Graphic format is a graphic file format that uses lossless compression algorithm to store raster images. It uses 2 stage compression methods. It is frequently used as web site images rather than printing as it supports only the RGB color model. So CMYK color images cannot be saved as PNG image.