Pixel Art is visual art or graphic media created electronically, and is composed of small squares known as pixels, rather than Vector Art, which is composed of mathematical equations. It is respected as a legitimate form of graphic art and design, just like any other form of visual art.
It has been used for several decades as the standard for creating Sprites, though it has found many more common applications in media as of recent. "The term pixel art was first published by Adele Goldberg and Robert Flegal of Xerox Palo Alto Research Center in 1982. The concept, however, goes back about 11 years before that." 
PNG, also known as Portable Network Graphics, and GIF, also known as Graphics Interchange Format, are two types of image formats that are commonly used to save images. GIFs are able to allow certain images to be set into moving images, moving one frame at a time, much like animations. PNGs, however, cannot, as they are used for single-frame images. JPEGs, on the other hand, aren't used as much as any other file format, as they cause images to have dithering (display a color image in such a way that there appears to be more colors in it than are really available) on them. They change the images by making the colors appear incorrectly.