Bittrex is a centralised exchange and is hosted by a provider. In a decentralised exchange, one does not rely on a third party service to hold the customer's funds. Instead, trades occur directly between users (peer to peer) through an automated process - Bitshares is an example

Bittex does not have an official app - there are some thirdparty ones that helps you connect to bittrex accounts - but I avoid those since I'm not certain how secure they are..