The difference between the two is that bitcoin is the main token or currency while etherium is just an alternative coins. But i do think that of all the alternstive coins existing in the market etherium is the best rival of Bitcoin. Since it has been following bitcoins progress in the market.
Ethereum and Bitcoin differ a lot. If you are interested to get the detailed and curious information about the difference between these cryptos, watch it:
https://www.youtube.com/watch?v=BSBfuGRvfvw&t=1s