BitCoin is the best known blockchain. Ethereum is the “new kid on the block”, and it has its similarities and its difference with BitCoin. At first, Ethereum has running nodes, transactions, blocks, and a blockchain build by consensus:
The consensus is based on proof of work for each block added to the blockchain, like in BitCoin. But the internal structure of transactions, the state of world by block, are quite different. For example, there are accounts WITH STATUS, instead of unspent outputs as in BitCoin.
But the main, key difference, is that each account could be an smart contract. Each node has a Ethereum Virtual Machine that can run compiled smart contracts. A method in a smart contract could be invoked in each transaction. This new capability opens lot of new use cases.
Having smart contracts in Ethererum/RSK, is one of the motivation to connect both blockchain: BitCoin is limited, running scripts, and the smart contract world is new brave world to be explored.