Sideart is the pictures on the side of an arcade game.
Sideart was pioneered early on in the amusement industry, long before the first Computer Space and Pong machines were ever built. Early sideart, (on pinball machines, slot machines and similar games served the purpose of drawing people toward what would otherwise be a rather boring black box. Early sideart was almost always hand painted, (the industry later started using stickers).
Painted sideart was used up until the early 1980's. The painted games have a very distinctive look that later games cannot match. The actual painting process varied over time, but some early games were hand painted, (some pinballs were even painted without stencils), later on the process became fairly automated.
A game with good painted sideart is what most collectors are interested in. The condition of the sideart will be one of the largest factors in the value of a classic arcade game.
Painted sideart is notoriously hard to restore or refinish. The games often use nonstandard colors, (and it is difficult to color match a faded 20 year old paint chip). Reproduction stencils are available for some of the most popular games, (but the color matching issue still exists).
Most games with painted sideart, (or dedicated cabinets), were painted over in the early 80's, to be turned into different games. Sometimes it is possible to remove the top coat of paint, and get the sideart back. But this is very difficult, and may be impossible in some cases.
Games that used painted sideart include.
Painting arcade games proved to be too expensive and time consuming for the arcade manufacturers. So they started using stickers, (and some other similar methods of printing the art). The good thing about the newer sideart was that it was a lot more colorful, (most painted games didn't exactly use a lot of color). But the sticker sideart is not as durable as the older painted sideart was. It was quite easy for a 10 year old kid to peel half the sideart off of ones of these games in a couple of minutes.
The major advantage to the modern sideart is that it is easy to come by. Unless you have a rare game, you will probably be able to find a replacement, (either NOS or reproduction). The same cannot be said for the painted games.
Reproduction Painted Sideart
Several companies now make large vinyl stickers that reproduce the painted sideart from some of the most popular classic games. These look great, but they do not look original at all. They are too glossy. For a good comparison you should try to find an arcade that still has a Ms. Pac-Man, and that also has one of those new Ms. Pac-Man/Galaga combination games. Compare the Ms. Pac-Man art on each one, (the new game uses stickers). You will see a remarkable difference.