cryptocurrencies is the umbrella term for all cryptographic based currencies, thus including bitcoin.
then beneath that it separates into bitcoin litecoin and namecoin (the three main first gen) and altcoins, where most the other cheap pump and dump altcoins exist which have atleast 50% code copied from the first gen.

be aware that things like ripple, is not in the cryptocurrency category at all as its just a mysql database. just like facebook credits, world of warcraft gold, applepay, paypal. these are all part of their own categories

and then... digital currency encapsulates them all..

so going from biggest embrella to lowest entity it should look like this