Droning on: where can you legally fly your drone?

Is it a bird? Is it a plane? No, it's probably a drone. Quadcopters, multirotors, and other forms of unmanned aircraft vehicle (UAV) are taking off in a big way.
We're not talking about a novelty gift that self-destructs on impact with the lamp by the end of Christmas day, or serious hobbyist tech that costs thousands to put together. Those are toys or for serious, long-standing enthusiasts.
We're here to chat about affordable drones, equipped with cameras and capable of flying anywhere. These are within grasp of the general public now, and who wouldn't want their own flying surveillance robot?
The rise of consumer drones has been rapid. Prices are falling, built in GPS and stabilization systems are making them easier to pilot, and you can use them to capture stunning video footage. Regulators have been left frantically writing a new rule book to police the skies. So, what exactly are the rules so far?

What kinds of drones are there?

Before we get to the rules, it's worth pointing out what a drone actually IS. Drone has become a bit of a blanket term that covers a lot of different aircraft types, but what they have in common is that they are unmanned.

In the consumer market, they're typically remote-controlled by someone nearby, though they may have the ability to fly autonomously.
They fly using rotor blades, like a helicopter, and the majority of the civilian drones out there are quadcopters with four sets of blades, which aids stability.
They also come in a wide range of different sizes. Many of them have cameras built-in to record the flight, some have the ability to accommodate action cameras, like the GoPro.
Prices start around £40, and you can get something really sophisticated for well under £1,000. As your budget goes up, so do the capabilities of the drones.
Some can navigate themselves, using GPS. Many drones have clever safety features; for example, if they lose contact with the remote control they may return to the last spot where they were in contact or find a safe spot to land.

You can also get drones that route a live feed from their camera to your smartphone in real-time.

So what are the actual rules?

The Civil Aviation Authority (CAA) makes the rules in the UK, and the first thing to note is that you need permission from the CAA to fly a drone that weighs more than 20kg, or to fly a drone for "commercial use".
As long as you aren't being paid, and your drone, with any equipment it may be carrying, weighs less than 20kg, you won't need to obtain a license.
Before you take to the skies, there are some other rules to bear in mind. You must fly the drone "within sight", with a limit of 400 feet in altitude and 500 metres horizontally.
You also have to stay at least 150 metres away from congested areas or large gatherings of people, and at least 50 metres away from people, vehicles, and structures that are not under your control.
You should also keep privacy laws in mind. Filming people without permission when they have a reasonable expectation of privacy is a crime. In practice, what you get away with depends largely on who sees you and whether they complain.

You might reasonably expect to be able to fly a drone in your back garden, but fly it in a busy local park, and you could be breaking the law.
In the US, the Federal Aviation Administration (FAA) has made similar rules. You can't fly above 400 feet, the drone should be within line of sight, and you shouldn't be within five miles of an airport, or near controlled airspace, such as over a stadium. Flying a drone for commercial purposes without permission is illegal.