Zuma is the name given by the players to the Scroll Workshop that can be found in the 5v5 Borderland maps (And in it's variations; Apocalypse, Native Land and Switcharoo). It's a small minigame, based on the fairly famous and known Luxor. The objective in this game, however, is not to just make all the balls dissapear, but to paint the picture used as background by making the balls of certain colors dissapear.

In most MOBAs to date, player are forced to either share a lane (if there's a jungler, only one) or to share jungle. In Prime World, however, it is posible to have three lanes with only one player, a jungler, and a zuma player. Now, while zuma players don't actively fight for most of the game, they are able to send scrolls to their allies (or themselves, but you shouldn't do that most of the time). By playing zuma, you will fall behind in prime in comparison to the rest of lanes; but you won't fall too far behind. Usually, if you're fast enough, you won't be more than three levels behind. And with the help of your scrolls, jungler and laners will be able to farm, be safe or even get kills much safer. A good zuma player can turn the tide of a game with a few scrolls to some players.

There's currently five different scrolls:

Bird Scroll[primeworld.gamepedia.com]
Divine Scroll[primeworld.gamepedia.com] The healing it does scales over the course of the match.
Starfall Scroll[primeworld.gamepedia.com] The damage it does scales over the course of the match.
Light Scroll[primeworld.gamepedia.com]
Gemini Scroll[primeworld.gamepedia.com]

The scrolls you will want to give more copies of are the Divine Scroll (usually a quick one to the jungler so he can do all camps safely and one more to each lane so they can farm freely), the Light Scroll (one to each lane and the jungler, two or three extra to the guy they're going to focus on teamfights) and the Bird Scroll (this one you will want to give it to your initiator, no matter where he is).
Gemini scrolls and starfall scrolls are situational scrolls. If you have strong auto-attackers, Gemini is a fantastic scroll to give them, though you can give it to casters too for a nice 20% Cooldown Reduction.
Starfall scrolls are used mostly to clean jungle camps faster, but if you know your team is going to gank a lane, you can try to do a fast Starfall Scroll and give it to one of them so they can safely finish the enemy off. Specially useful against heroes like Whisp and Eraser, since the Starfall Scroll also hits invisible targets.

The answer is quite simple. For as long as your team doesn't need you. If you see the enemy team ganking an ally and you just finished/started a scroll, you might want to get out of zuma and teleport. If they don't turn back, or if you can kill them (with the help of someone else, as you will be fairly weaker), finish it. Otherwise, cancel the teleport and happily go back to zuma. You will have succesfuly saved an ally just by wasting 8 seconds of your zuma time.
When you're not fighting with your team, you should go back to zuma and keep producing scrolls so that your team is stocked up and can slaughter the enemy team in teamfights.

Well, I am of the opinion that you can zuma almost anything, but the ideal heroes to zuma as are:

Maiden/Nymph Probably the best zuma hero in the game, has everything a supportive character could need.

Healer/Priestess A great zuma hero, since she has an auto-invulnerability spell that heals allies arround her, allowing her not to be picked off due to her lack of levels.

Quarrier/Rumbler Your typical tank. The one you either love or hate. He has an invulnerability skill that heals him and a good engage/finisher ultimate.

Eraser/Assasin Although this character is not used much in high ratings, due to most of his power coming from invisibility, it works wonders in the mid and low ratings, and it can be pretty fun.

Shadow/Whisp A permanently invisible hero. Her ganks are pretty nice once she gets her knockback, and she can nuke targets down if she has a starfall scroll (to clean the lane) and her ultimate. The usual strategy with her is to zuma for a bit, give yourself one or two starfalls and go ganking.

Those are the most usual ones. But you can play pretty much any hero in zuma. For example, as a main zuma player, I play Duelist/Prince of thieves in zuma, and it works well if you know what you're doing :) So don't worry much about a hero you want to zuma with not being here; find your own way to play, test things for yourself, and above anything, have fun!