What exactly is a chroot? Is it similar to a simultaneous dual boot?

I feel like nobody gave a full technical explanation, so here goes.

To understand a chroot you first have to understand the split between kernel (The Linux in GNU/Linux) and userspace (the GNU in GNU/Linux, or possibly something else, like busybox.)

The kernel controls all the hardware in your computer. It also provides the APIs for file access, networking, and so on, and controls which software programs are running. This all forms an abstraction of the computer, which is presented through an application programming interface (API). It doesn't do anything at all on it's own though, except maybe boot up to a black screen. Not even a shell prompt.

The userspace is everything else. All the software that you run on your computer. Actually the kernel only runs one userspace program directly, init, which is then responsible for starting everything else, like shells and desktop environments. Userspace also includes libraries, which generally start with libc, upon which all the other libraries build.

So, with that in mind, the concept of a chroot is simple. It just changes the root directory of the unix filesystem to a different one, just for whatever command you choose to run in this context. This is usually a shell which you can launch other software, much as the kernel only launches a single command directly. This new context can have a different set of userspace programs and libraries. The same kernel is running both sets of software, so both systems can use all the hardware resources, but (barring security bugs) the nested chroot cannot access anything from the primary filesystem. It has it's own version of /etc for configuration, it's own /lib for libraries, and it's own /bin, /usr/bin for programs.

You should be aware that the hardware devices are shared. So, unlike a virtual machine, if you format /dev/sda from inside the chroot, you will format your real harddisk. This is because the device nodes in /dev are a direct kernel interface, therefore they mean the same thing inside and outside the chroot.

One other thing: it is possible to give the chroot access to the outer filesystem with bind mounts. If you use a chroot building tool it is possible that it "helpfully" mounts /home from the main system inside the chroot. This is not a copy, it's the same filesystem, and in this case any changes you make inside the chroot will be performed on the original. So I recommend you build chroots manually until you are comfortable with how they work.

Other than these two potential problems there isn't much that can go wrong with a chroot, as it is mostly a self-contained system that will only be started when you ask for it.

A Non-Technical Explanation

Let me try to explain chroot in terms of Windows. In windows the "root" of the boot partition is called "C:\" In Linux it is called "/". choort (temporarily) allows you to make some other folder/partition/device the root partition. If Windows had a choot command it might have worked as follows.

Imagine you have a computer with two partitions or drives with two versions (or copies) of Windows installed. Lets Call them WinA and WinB. When you boot WinA, its root becomes C:\ and WinB may appear to be in D:. When you boot WinB, C:\ refers to the partition where WinB is installed and D:\ is where WinA is. Now let's say you want to make some changes to both WinA and WinB by running a program z. When you run z it does all the changes you want to the system that has the root C:. In the Windows world you have to boot WinA run z and then boot WinB and run z. chroot allows you to make the D:\ into C:\ without rebooting. So you can boot WinA run chroot to make the D: the new C: and run program z so that changes are made to the WinB which resides in D:\ instead of WinA, which is in the original C:.

Now let me give an example of how I have used chroot. Once my Ubuntu became unbootable. It turned out that the grub needed to be reinstalled. This is easy if I could boot Ubuntu and run the command that installs grub. But I needed to do this from an USB. So I booted Ubuntu from a USB drive. If I issued the command to install grub I would install grub to the USB. So I needed to mount the partion in the hard drive where the Ubuntu with broken grub is installed and use chroot to point to that mounted partition. Then I reinstalled grub and grub got installed in the hard drive where it should be.

chroot has nothing to do with dual booting. The idea behind chroot is the ability to switch one program, or one shell, etc., to a new root directory, allowing you to have multiple "systems" at the same time. "systems" is in quotes because there is still only one Linux kernel running. This other "system" is an additional set of the code tools, home directories, etc. So, if you are on an x86 machine, and are trying to build some software for ARM on your machine, you could create a chrooted environment whose /proc tells the software that it's on ARM, an ARM GCC toolchain, etc. (There are many programs that do that). The other thing chroot can do is some very limited sandboxing, that combined with something like SELinux and basic Linux permissions, could create a relatively secure sandbox, in which this application thinks it's on a different system. Another use of chroot would be testing. If you have an application that does different things based on some system directory, you can create a chroot to test it in.