Altcoin is a cryptocurrency term for any coins or tokens other than Bitcoin.
Bitcoin is the first and original cryptocurrency and after it gained in popularity, several clones of Bitcoin started to appear around the year 2011. One of the most popular and resilient of these clones is Litecoin which is today a top 10 coin with around $1.7B marketcap.
Altcoins should not be viewed as mere copies of Bitcoin as some altcoins or tokens have uses cases other than the simply being a currency like Bitcoin.
Some other uses that altcoins have are:
Privacy Coins – provide more privacy and anonymity than Bitcoin ex. Monero, ZCash
Blockchain Coins/Tokens – provide access to blockchain services or allow use of a blockchain. Examples are: ETH and EOS
Utility Tokens – provide access to services, Apps or dApps. Ex: BAT, BNB
Stablecoins – meant to buck the trend of cryptocurrency being volatile by pegging their value to the USD, other fiat currencies or a basket of precious metals or commodities.
Asset backed coins – coins whose value is backed by a valuable asset.
As the use cases and penetration of cryptocurrency, blockchain and tokens increase in number, it is entirely possible that the number of altcoins will explode as tokenization revolutionizes the way we live our every day life.