Photoshop User Guide has a chapter on "Getting Images into Photoshop" it
describes the differences between bitmap (grid or "raster") images versus
vector graphics (consisting of lines and curves). The chapter then goes on
to explaining resolution and images size and how scanners, monitors,
printers etc. display the information and how to change display. Even if
you are not working with Photoshop the background of how images are
processed digitally might be helpful. Adobe web page also has help pages.
You might check out: