Both the hardware and system software are created by Apple.
Apple limits the functionality of third-party softwares.
The third-party software makers must send their product to Apple, and without the permission of Apple, they cannot sell them openly.
This attitude is called as vertical integration.

Android is not vertically integrated so you can create the third-party application freely.
You can sell your apps freely without the permission of Google.

There are pros and cons of vertical integration.

Using iPhone, you don't encounter so many malwares and viruses.
This is because the software is closed.
For Android, you encounter rather more malwares and viruses.
In other words, iPhone is safer.

But Android is more free.
You can hack the Androids more than iPhones.

So if you are hackers, know well about IT, and are brave, you might prefer Androids than iPhones.
On the other hand, if you don't want to spend much time to learn about cell phones security and are coward (like me), you might prefer iPhones.

But if you love iPhone design and UX and you still want to hack it freely, you might want to jailbreak it.
To jailbreak is to apply a patch to your iPhone and install the softwares and tweaks that Apple doesn't permit.

Is jailbreak illegal?
So far, it is not illegal due to the American federation laws.
But Mr. predident Obama stated that he would might make it illegal.
I don't know whether it would be legal forever.

I jailbroke my iPhone.
Because the official Japanese input system take too much space on the screen.
Apple made it worse in iOS5.
So I apply the tweak to make it in iOS4 style.
This is only one reason that I had jailbroken my iPhone.
If Apple would permit me to make it back in iOS4 style, then I won't jailbreak it.

Apple dislikes the jailbreakers.
So everytime when they create the newer version of iOS, they would make the jailbreak impossible.
So the jailbreak patch creaters work very hard to make the jailbreak possible again.

How can you jailbreak it?
Please search evasi0n7.
Note that I don't recommend you to jailbreak your iPhone.
Please do it with your own risk.

I think the creaters of iOS in Apple are jailbreaking their iPhones too.
If some cool tweak had been created by the jailbreak programmers, then Apple implemented the same tweak to the official iOS.
So I hope Apple won't prohibit the jailbreak perfectly.