Dash (DASH) is a privacy-centric digital currency with instant transactions. It is based on the Bitcoin software, but it has a two tier network that improves it. Dash allows you to remain anonymous while you make transactions, similar to cash.
With Bitcoin, transactions are published to the blockchain and you can prove who made them or to whom, but with Dash the anonymization technology makes it impossible to trace them.
This is important because the blockchain is accessible to anyone with an internet connection - a significant drawback for those don't wish their transaction history and balances to be publicly available. Dash does this through a mixing protocol utilizing an innovative decentralized network of servers called Masternodes, avoiding the need for a trusted third party that could compromise the integrity of the system.
Dash transactions are almost instantly confirmed by the Masternodes network. This is a great improvement on Bitcoin's system, where confirmations take much longer because all the work is done by the miners.