The main difference is in the screen resolution (that is, the number of pixels on the screen). The more pixels you have, the more detail the monitor can display.
Some standard formats and the number of pixels (width x height) are:
VGA: 640 x 480
SVGA: 800 x 600
XGA: 1024 x 768
SXGA: 1280 x 1024
SXGA+: 1400 x 1050
UXGA: 1600 x 1200
QXGA: 2048 x 1536
Another minor difference is the "aspect ratio" which is the ratio of width to height of the screen. Most standard formats have an aspect ratio of 4 to 3, except for SXGA which has a ratio of 5 to 4.
Wide screen formats are prefixed by a "W" and have an aspect ratio of 16 to 10 or 16 to 9. For example:
WSXGA: 1440 x 900
WSXGA+: 1680 x 1050
WUXGA: 1920 x 1200