Bitcoin (BTC) is the preeminent cryptocurrency and the first to be used widely. However, hundreds of cryptocurrencies exist, and more spring into being every month.
Altcoins also sometimes differ in the method of verification used to authenticate transactions. While some altcoins use Proof-of-Work systems, others use Proof-of-Stake consensus, which replaces miners with validators. Proof-Of-Stake mining requires a lot less energy and fewer resources than Proof-Of-Work systems since Proof-Of-Work miners have to do much more ‘work’ to mine blocks.

Savvy cryptocurrency users are therefore maniacally protective of their private keys, typically storing them in multiple digital locations — although generally not Internet-connected, for security purposes — and on paper or in other physical form.
As noted elsewhere by a prominent author in the area [25], there is not a widely accepted or consistent definition of the term “bubble”. The GDASF test used here assumes a bubble is any time series interval which deviates from a random walk to become explosive.