Parity is an Ethereum client, written from the ground-up for correctness-verifiability, modularisation, low-footprint and high-performance. It utilities the Rust Rust programming language, a hybrid imperative/OO/functional language with an emphasis on efficiency. You will find here a chart showing the efficiency of this client compared to geth or eth.Parity is professionally developed by Ethcore.
As pointed by Vitalik Buterin in this twit, it is highly recommended to connect to Ethereum with this client, especially with minimum version number 1.2. It seems Parity is gaining traction among the community and may become the upstream ethereum client.