Bitcoin (BTC) is the bitcoin implementation with the most security, and hashing power; a store of digital value and transfer allowing anyone to send money to anyone in a trustless and secure manner.
The main version of bitcoin which strives to bring peer 2 peer cash to the world while keep the blockchain ledger size as small as possible. Keeping the ledger small ensures everyone can run, verify and help secure the blockchain without having to trust any other entity.
Keeping the blockchain small also makes the blockchain much harder to censor and control, as the data requirements are small enough to get through any resistance.
When a merchant 'Accepts Bitcoin' there's a good chance they also accept other cryptocurrencies such as Bitcoin Cash (BCH), Ethereum (ETH), Litecoin (LTC) and possibly other crypto assets. Try them all out and experience using your cryptocurrency as cash.