Hot air balloons are the only vehicles able to fly, outside of glitching other vehicles to do the same. It is easy to pilot, with a little arrow displaying the wind direction whenever you start going up to indicate the current direction it will head towards. Its speed changes based on altitude, being faster if it is higher up in the skybox limit. It can also be fireproofed just like any other vehicle, with the exception of the balloon itself.
Uses
Apart from being a handy way to travel and one most people won't be able to contest until you land, it is also used to create mobile skybases with cactus and other techniques to stick objects onto it.
When facing someone who has a balloon (even if it is being used as support for a skybase), you can use a bow with fire arrows to lit their balloon on fire. Since the balloon itself cannot be fireproofed, they will go down regardless of what they do as putting out the fire is not possible either. Grenades, if in range, will also do short work of the balloon.
The balloon can be used for a variation of the mobile skybase, as mentioned in the above tip. The easiest way to get one going is to grab however many cacti you need, segments/composts, and chain them together with the cactus so you are able to place needed buildings on top of it. All that's needed for a self-sufficient base is a well, composts, and wheat along with a collection of any fruits you like so you're able to make food whenever you need to.
A more complicated and time-consuming, but safer way of creating a skybase can be seen in the video below. It uses composts and tables along with a balloon weld to be able to attach composts to it, which gives the very significant benefit of the base not being draggable as well as impossible to ignite (if fireproofed); the only way to destroy such a base is to ignite the balloon itself or by using explosives, both of which are unlikely to work unless you are extremely close to the skybase or above it.
Keep in mind that this version of the skybase requires at least two people to make work, or an alt account: