FreeBSD jails are like a really mature, full-featured version of LXC as opposed to "just a chroot". In addition to being chroot that provides real filesystem isolation without many of the security issues of a Linux chroot, it also has CPU and memory limits, disk quotas, network isolation, root privilege isolation, all the magical ZFS goodness (provided you're running the jail on ZFS). It's really, really nice.
Running ps in a jail will show the processes in the jail only.
Processes in the jail will only see network interfaces and other devices that have been explicitly exposed to the jail.
As others have said, it's like a vm with no virtualization overhead. You can set up jails with the entire Freebsd fs hierarchy so it runs like another host with its own users. Note that even the root user in a jail is not the same as the real root user. You can then use pkg to install packages within the jail too.
chroot (often referred to as a "chroot jail") limits a process to a certain subset of the filesystem - e.g. you could limit a httpd process so that it can only see /var/www, and it would not be able to see anything outside that, so if there was a security compromise of the web server, an attacker would not be able see anything outside that folder tree.
A FreeBSD jail is a like a lightweight virtual machine, and is very similar to a Docker container in Linux (though it has been around for about a decade longer than Docker). It provides isolation for processes etc., but uses less resources than a full virtual machine. It is limited in that it has to be the same operating system as the host.
