Mining rigs are computers built specifically for the purpose of mining POW crypto currencies. These computers have all the standard parts like a common desktop computer : CPU, memory, motherboard ... but they contain multiple high end GPUs (graphic cards) and they also need a special case, as 6 or more GPUs wont fit in a standard computer case.
Open cases are used in most cases, to assure proper cooling as the GPUs produce a lot of heat when in operation. In order to reach the desired performance, lot's of modding is required (custom bios, specific power and memory settings).
A mining rig consumes a lot of electricity, so they need to be operated in locations with custom/favorable electricity rates, in order to reach profitability.