This is a typical 1U 19" rackmount server.
The inside of rackmount servers is highly customized, so don’t expect to find many standard components, with the exception of memory modules, processors and a few expansion cards. The height of 19" components is specified in so-called "units", and you can find computers in 1U, 2U and 4U form factors for 19" installation. There are even larger systems, but these are very proprietary. The example above is a 1U dual Xeon socket 771 server system by Supermicro.
It’s not just servers that are designed for 19" racks: many other products can be fit into these racks as well: network switches, routers and firewall systems, patch panels, studio audio and video components, uninterruptible power supplies (UPS), storage appliances such as NAS servers, phone appliances and more.
Pay attention to the maximum heat dissipation for rackmount servers, because high density computers can exceed typical specifications easily. Since 19" racks are closed, they need to be able to dissipate several kW of heat power via fans or other cooling solutions.