The term resolution refers to to the amount of information in an image.
Devices such as digital cameras, printers, scanners & monitors all have a particular resolution.
Image resolution is measured by the amount of Pixels Per linear Inch (PPI) or Dots Per Inch (DPI) if referring to print resolution.
The more pixels per inch the higher the image quality (and the larger the file size will be) .
High quality print resolution is typically 300ppi. 300ppi is roughly the limit of what the human eye can distinguish.
Monitor resolution varies greatly depending on the device (from a 19ppi TV to a 340ppi mobile phone). Find the PPI of your monitor in this Wiki entry.
Unlike when printing, an image will be displayed on a monitor pixel to pixel so will change size depending on the devise.