Nothing divides the crypto-currency community more than the division between Bitcoin and Bitcoin Cash. On August 1, 2017, a group of developers in the Bitcoin community did a hard fork. This means they created a version of Bitcoin that was incompatible with the existing system, but they implemented it in such a way that all pre-existing Bitcoin transactions were valid. At that point, the blockchain that defines Bitcoin split into two paths. All old transactions existed on both, so anyone who owned Bitcoin had valid coins on both systems. It should be noted that this is very different from starting a new crypto-currency from scratch and starting a new blockchain.
Both the unchanged fork (Bitcoin) and the new fork (Bitcoin Cash) consider themselves to be the “true” Bitcoin, whatever that means. Both paths are implementing changes to address technical challenges that face all crypto-currencies, so only time will tell which fork the community decides is the most useful and therefore most valuable. While many people sit on the sidelines of this battle content to see which system proves itself more, the developers, miners and evangelists for Bitcoin are divided. Like any Internet discussion, it immediately became an us-vs-them with extremists ridiculing anyone who disagreed with them, but for the larger marketplace, it just means that we have two systems to act as a proving ground for technological improvements and market adoption.
For more info, see these writeups: