A job server is a large computer placed in a datacenter. A job server has anywhere from 8 up to about a hundred CPU cores and several hundreds of gigabytes of RAM. A server combines low latency internal storage with fast access to shared storage through high-speed networking inside the datacenter.
A job server has high quality redundant components and lives inside the controlled environment of the datacenter. A job server is always on. Being inside the datacenter protects the server from most network failures and virtually all power failures. This makes job servers very reliable and allows them to run uninterrupted for months. This stability is essential when you have large computations that must run uninterrupted for days or even weeks.
The typical work pattern when using job servers is to keep most of the workflow - like development and visualization - on your laptop, but offload the heavy computations to the job servers. This works best if you can create jobs that do not require user interaction.