Bitcoin was the first true cryptocurrency and has been in circulation since 2009. Ethereum is a far more recent development, going live in 2015.
In the time between Bitcoin and Ethereum’s release, lots of other cryptocurrencies emerged. Mostly, however, they were limited to trying to improve on aspects of Bitcoin’s performance – for example, increasing speed of transactions or improving the security or anonymity of transactions.
Ethereum is certainly faster than Bitcoin – with transactions typically settling in seconds, rather than minutes. But it also takes things further. While still blockchain based, and operating as a store of value, its fans and evangelists see it as a platform for distributed computing, which comes with its own built-in currency, called Ether.