Jails are the operating system level virtualisation of FreeBSD. Jails allow you to run multiple isolated user space instances using the same host kernel.
Jails enhance the chroot concept, which in general is used to change the root directory of a set of different processes.
Jails have a directory subtree, processes are not permitted to escape from.
Jails have an own hostname and own IP address.
Jails have a path name of an executable which will be run inside the jail. The path name is relative ti the jail's root directory.
Jails also have their own root account which isn't allowed to perform any operations to system outside the jail and an own set of non-root users.