In Ethereum you can write your own "contract" or a "currency" which is pretty much the same thing.
Ethereum uses one blockchain with particular properties - Ethereum Block Chain designed specifically to create Cryptographic Contracts. To make transactions on Ethereum Block Chain you need to use Ether - Ethereum Crypto Currency
Eris main purpose is to build your own blockchain - it is a block chain design and define any property ou need for a particular case. For example you need to record votes of the members of the organisation, and instead of storing the votes on the server, such as, for example, while using One Click Orgs or Loomio, it will be stored on the Block Chain - which means no one person would be able to change this history.
Another example will be saving histry of the changes in documents stored on the organisaion server.