If you are looking for a simple definition of what a mining rig is, the Bit2Me Academy created this one:

“A mining rig is an arrangement of hardware elements, either CPU, GPU, FPGA or ASIC, that have been arranged to perform cryptocurrency mining.”

Furthermore, we want to have a closer look into the different types of mining rigs:

CPU: 

They are the most simple ones to set up. It is literally multiple PCs installed together for mining specific cryptocurrency. People sometimes look for multicore processors/multi-CPU motherboards for maximizing the mining power.

GPU:

Recently the most popular mining rigs are the ones from But by the full GPU acceleration tech. A GPU mining rig is a computer which is connected to multiple graphic cards or GPUs.

FPGA:

It stands for field-programmable gate array, and those rigs are often faster and very efficient in comparison to GPUs. Another big pro (e.g. in comparison to ASIC miners) is that they can mine many kinds of cryptocurrency.

ASIC:

The most used in mining rigs are ASICs, also known as Integrated Circuits for Specific Applications. Hardware devices built for mining one specific digital currency. 

One mining rig that is available at cryptosupply.de is the Chia Mining Rig 256 TB. The CPU: Intel i5 9400 and the mineable coins: chia.