The upright is by far the most common arcade game form factor. It was used by the first Computer Space machines to ever be built, and has enjoyed tremendous popularity ever since. It has only recently began to wain in popularity in the last few years, as the industry has begun to concentrate on sitdowns and bartops.
Upright machines are large free standing machines that are generally around six feet tall. They have player controls about halfway up the machine, with a monitor above them. The coin mechs are usually located down low on the machine, but a few early titles took money in directly from the control panel.
These machines are designed first and foremost for easy conversion, that vast majority of them use nothing but industry standard parts, and can be converted into another title in a matter of hours (or minutes).
There are three basic sizes of upright machines. Standard machines are just under six feet tall, and are around two feet wide. These machines usually feature 19" monitors, and generally have control panels that are not any wider than the machine. Four player uprights are much like the standard ones, but they have a large protruding control panel that four people can just barely squeeze around. Deluxe uprights are a rather new invention. Most newer games use these cabinets. These games usually feature 25" or larger monitors (some go as high as 50"), and have huge control panels that stick out way past the machine.
There are literally hundreds of different designs for the standard upright cabinet (a collector can usually tell you what game was originally in one merely by the shape of one). The newer deluxe cabinets only come in a few distinct styles that are used by all the manufacturers (there are probably only about a dozen of them).