There are two main graphic types, bitmap and vector graphics.
Vector graphics describe an image according to its geometric characteristics and can be resized without losing quality.
In Android, we’re more interested in Bitmap images which are stored electronically as a map of bits.
Another electronic image term is the pixel. Sometimes bits and pixels are interchanged and a bitmap can refer to a map of pixels, where each pixel represents a colour.
Bitmaps are resolution dependent which means they contain a fixed number of pixels so they will lose quality if you change their size.