I always hear about backdoors and I understand the main purpose but I have some questions:
In what kind of software/webapplication/OS can I found them ?
Literally anything.
How can I recognize one ?
By reverse engineering the software and carefully analysing it for flaws in authentication and access control, as well as issues with memory access in native applications. It's the same process by which you'd find any other vulnerability. This is not a trivial task, and entire books have been written on the subject.
If you're looking to know the difference between a generic security vulnerability and a backdoor, the difference is the intent of the programmer who put it there. You have to find evidence and use intuition to identify if it was purposeful or not. Usually this is not a yes/no answer.
How do I prevent them ?
You basically can't. You just have to expect software to be broken and have a plan to (a) keep things up to date, and (b) deal with it should there be a breach. It's not about if, it's about when.
Is the analogy good to compare them to have root access (in a Linux way) ?
Not exactly. A backdoor is anything that provides a lesser-authorised user to gain access to something they shouldn't. The backdoor might allow full access to an unauthenticated user, or it might allow some limited access to an unauthenticated user, or it might allow an authenticated low-privilege user to gain access to something at a higher privilege level.
Any more relevent informations about them.
There isn't much, really. Backdoors are just intentional faults put into code to give someone access outside of the normal security model.
Backdoors are part of the software itself. They are placed in the source code (or hardware) from the start. It allows the original coder some sort of access through their software or hardware. With that in mind:
Due to the nature of backdoors there is no kind of software that specifically has backdoors. They can be in any piece of software or hardware on any platform.
If you suspect that a piece of hardware or software has a backdoor you'll need to analyze it. Reverse engineer the binary to find the anomaly. This can be time consuming. A lot of the techniques talked about in this question and answer apply to analyzing a piece of software.
Backdoors are deliberately inserted into computer systems by the originator. There is no prevention. If you find a backdoor it might be possible to patch it yourself. However, with the increased amount of code signed binaries this might be prove to become more difficult.
While backdoors can give root access it's not really an analogy. A better analogy would be asking a locksmith to install locks in your home, and while he's there he installs a special lock that while it opens with the key he gave you it also opens with a key he has.
The important thing to remember about backdoors is that they are deliberate. They aren't bugs in the code. The developer/designer placed a vulnerability within the system on purpose. Some backdoors can be easy to spot. For example, you buy a router and you notice that there is an extra admin account that you didn't create. Others might require specific exploitation of a software vulnerability to leverage it. They come in all shapes and sizes (just like other vulnerabilities).
An Example of a Backdoor in an Open Source Project
Web Backdoor Example
