A mining rig is a computer system used for mining bitcoins. The rig might be a dedicated miner where it was procured, built and operated specifically for mining or it could otherwise be a computer that fills other needs, such as performing as a gaming system, and is used to mine only on a part-time basis.
Warning: GPU mining is not very profitable (if at all) anymore, and even if you have free electricity, GPU rigs will likely never pay for themselves at this point!
ASICS for Litecoin
An ASIC is an Application Specific Integrated Circuit. They are specially designed pieces of hardware for performing the hashing algorithms necessary to mine a specific coin and verify hashed transactions. These pieces of hardware are designed and manufactured to perform necessary hashing and nothing else.
With the increasing valuation of Bitcoin, in fiat terms, the economic viability of ASICs begun to make sense some time ago. The development of ASICs to do SHA256 became a reality. There were issues along the way with bringing them to market, but ASICs have arrived.
With the increasing valuation of any coin the same is true: ASICs become a viable option. It is difficult and some would say impossible to avoid ASIC development in the face of viable economic incentives.
ASICs are considered undesirable for a variety of reasons, not the least of which is that they concentrate mining power too heavily and diverge from the original ‘one cpu one vote’ intention of Satoshi.