OK, so your sub is comfortably restrained and squirming around contently but something seems missing... it's time to gag them. But just what kind of different gags are there?
This is by far the most popular kind of gag, and comes in several varieties. Most simply have a rubber or leather strap going right through the rubber ball (which is usually pink or red) which then goes around the submissive's head. Head harnesses take this idea further, and have various straps going below the submissive's chin and up between their eyes to the back of their head, meaning the ball itself is less likely to be moved out of place. These are sometimes lockable with small padlocks for even more fun.
Particularly popular amongst pony girls and pony boys, for obvious reasons. These are also available as head harnesses, some even going as far as to have blinkers to restrict the wearer's field of vision.
This has two main advantages or disadvantages, depending on your point of view: you can command your sub to lick various parts of your (or someone else's) body while they're still unable to speak properly, and when they're not doing that, they tend to drool all over themselves in a way that has a knack of running straight down their chest. Unfortunately, it seems to be a lot easier for this to fall out of place.
A rubber gag which has a hand pump protruding from it. The gag is inflated once it is inside the sub's mouth.
Similar to a bulb gag, but with two extrusions, one either side, that fit between the teeth and cheeks. This can apparently be hard to keep in the mouth, which again can be a disadvantage or an advantage depending on how you're playing.
Not recommended. Fun during play, but not so fun when you're gently trying to take it off afterwards. It works best when something soft is already carefully stuffed into the sub's mouth.
Scarves or, at a pinch, tea towels can do the trick. These work for some people, though speech isn't as impaired as it is with most other gags. The scarf type is aesthetically pleasing, and they can also be used as blindfolds.
Very scary looking. These don't make very effective gags and can be dangerous. Definitely not recommended.
There are other variations, such as phallic shaped gags and gags with funnels attached to them. Hopefully this has covered the main ones, though.