Altcoins are all cryptocurrencies launched after the success of Bitcoin. In general, they project themselves as better substitutes to Bitcoin. Altcoin is a combination of two words: alt and coin; alt meaning alternative and coin meaning cryptocurrency.
While Bitcoin was the first cryptocurrency, and remains the best known, it is now just one of hundreds of cryptocurrencies, all of which seek to improve upon Bitcoin in different ways. There are now thousands of Altcoin, new ones are appearing and some have disappeared to become deadcoin.
The website coinmarketcap.com lists all the crypto-currencies on the market.
