Every now and then you might need a Linux distribution for a CPU that you do not have. As an example you might want to run an ARM distro on an x86_64 machine. This is not only possible but quite simple. Here's how you would do it on Ubuntu or Debian.
First install the qemu-user-static package and create a subdirectory to hold your distro install. Then run the first stage install. Using Debian sid as an example:
sudo debootstrap --arch=armhf --foreign sid sid-arm
Once that has finished, copy the qemu binary inside this install. This makes it possible to run ARM binaries transparently on an Intel CPU.
sudo cp /usr/bin/qemu-arm-static sid-arm/usr/bin
Then finish the bootstrap operation.
sudo chroot sid-arm /debootstrap/debootstrap --second-stage
And now we are done. Chrooting in sid-arm and running uname -a prints this:
Linux userland 4.1.0-2-generic #2-Ubuntu SMP Wed Jul 22 18:19:08 UTC 2015 armv7l GNU/Linux
At this point you can use the chroot as if it were a native installation. You can install packages with apt, compile stuff with gcc and do anything else you might prefer. In theory you could even run it as a container with systemd-nspawn, but unfortunately qemu is missing some functionality to do that. The bug report is here. The only downside of this setup is that because the CPU is emulated, all binaries run a bit slow.