"Alt coins" is the collective name given to all cryptocurrencies that are not Bitcoin, which was the first major cryptocurrency. Like Bitcoin, alt coins are special kinds of virtual currency tokens that often reside on their own blockchains and represent an asset or utility.
They are mostly used to fundraise for crowd sales but they can also be used as a substitute for other things. Today, the total market value of all cryptocurrencies in existence is about $237.1 billion, with Bitcoin representing over 70% of that value.
There are currently more than 5000 different types of Crypto Coin available, though more are being released all the time.