What is cryptocurrency
Originally the bitcoin protocol gave birth to “digital scarcity”, digital scarcity was something we had never seen before bitcoin, we had always assumed that everything that was digital could be copied. Bitcoin and the blockchain does not stop something that can be reduced into bits and bytes from being copied. But is allows us to see where “copy” came from, what was the parentage.
Google gave me the following definition
a digital currency in which encryption techniques are used to regulate the generation of units of currency and verify the transfer of funds, operating independently of a central bank.
Why are there so many?
There are many because since the development of the bitcoin protocol others saw other ways of achieving “digital scarcity” which had different properties. The cryptocurrencies that followed are generally described as altcoins.