Bitcoin is the first blockchain based cryptocurrency and is the popular and most valuable one. The popularity of Bitcoin has given birth to hundreds of alternate crypto-currenies designed with various specifications and functions.
Some of these cryptocurrencies are clones of Bitcoins and few are forks. With so many cryptocurrencies out in the market, where new ones come out almost daily and old ones disappear seemingly.
In order to make a place and be considered as successful coin in the crypto market, these cryptocurrencies are ranked based on few criteria or values such as the market cap value, price, circulating supply of cryptocoins and maximum supply of crytocurrencies.
For more info please see https://mrbtc.org/about-33-cryptocurrencies/