Altcoins are a digital cryptocurrency similar to Bitcoin. The term altcoin stands for "Alternative to Bitcoin" and is used to describe any cryptocurrency except Bitcoin.
Ethereum is an example of a altcoin. It is a cryptocurrency similar to Bitcoin only the technology behind it is different therefor we call Ethereum an "Altcoin".
Next to Ethereum there are a lot of Altcoins. Some examples of them are Litecoin, Bitcoin Cash, Ripple, IOTA, Dash, NEO, EOS, Stellar, Cardano or Monero. You can view all the altcoins here.