How to download cydia and how to use it?

My friends told my that I should download cydia for my iphone. I want to know what it is? How to download it? How to use it? Is it illegal to use it? I have an iPhone 5s just in case that matters.

Many smartphone, tablet, and game console makers include a layer of Digital Rights Management (DRM) software on their products. This DRM exists either to limit the software you can run on it, or is there for security reasons. Jailbreaking is the process of hacking these devices to bypass DRM restrictions, allowing you to run "unauthorized" software and to make other tweaks to your operating system.

More technically, jailbreaking can be thought of as the process of installing "a modified set of kernel patches (the kernel being the supervisor of the operating system) that allow you to run unsigned code," as saurik explains. It also gives you root-level access, which is otherwise unavailable. The term "root" comes from UNIX, where it is the superuser account that has unrestricted rights and permissions to all files. This provides you with added flexibility, but it also has some inherent dangers (more on that later).

You can alter or install anything onto your phone or tablet such as UI hacks (like the 5-icon dock) or unofficial apps (like classic console emulators) that would not otherwise be accepted into the App Store. Saurik's alternative app store, called Cydia, is arguably the definitive repository of such apps or tweaks. Cydia is so ubiquitous in iOS jailbreaking circles that most jailbreak tools install it automatically.

iPhone hackers first coined the term "jailbreaking" in reference to breaking the iPhone out of Apple's iTunes "jail." The term has since been (somewhat erroneously) used in a more general sense to refer to similar hacks made to other platforms like Android, where it's also known as "rooting". In any case, the basic concept is the same, regardless of what you call it.

depends a little bit on who you ask, but as far as the US government is concerned, it is legal to jailbreak your smartphone. In 2010, the Digital Millennium Copyright Act (DMCA) recognized the arguments of the Electronic Frontier Foundation (EFF, the international non-profit digital rights advocacy organization) to add phone jailbreaking to a list of exemptions to anti-circumvention provisions. This DMCA exemption is up for review again this year, and the EFF and other groups like the Software Freedom Law Center hope to get the exemption expanded to cover hacking other personal computing devices--like tablets and game consoles--as well.

Some folks, particularly hackers and modders, insist that jailbreaking is a right and do it as something of a statement. They'll often compare it it souping up a car: Imagine buying a car with the hood welding shut so you can't modify it. Some say that's just not right.

Apple and some other companies, on the other hand, continue to plead with regulators to outlaw the practice. The best they can do at this point is tell users that jailbreaking defies their terms of use and voids the warranty. In other words, while you don't risk being arrested just for jailbreaking, don't expect any help from the Apple Store genius when your hacked iPhone has a problem. Jailbreaking is legal, downloading cracked apps from it is illegal.

FIND OUT IF YOUR DEVICE IS JAILBREAKABLE AND GET A LINK TO JAILBREAK IT:
http://www.jailbreak-me.info/