The first thing to know is that Bit coin is both a currency and a digital system. As a currency, it can be used for everything that any currency can use, but instead of having a government entity -like a central bank- that issues and supports it, it is based entirely on the digital system that was devised by its creator, Satoshi Nakamoto, in 2009: the block chain or chain of blocks. As a result, one of the most outstanding features of Bit coin is that it does not belong to any country or government; and since its creator is anonymous and decided that his invention was a free license, it does not belong to any individual or private company. Those who keep their platform running are the users themselves.
Bit coin is a currency just like the dollar, the euro, the yen or any other local currency that you can think of. Their uses are exactly the same. What makes a big difference with the so-called fiat currencies (those issued and backed by a central bank) is that it does not exist physically. It is a digital currency that only exists in the chain of blocks or block chain that supports it and due to a sophisticated process of verification (consensus) of transactions, it cannot be spent twice. Each bit coin - or portion of it - is unique, as each transaction is publicly registered (albeit without personal data) in a large digital "book" of accounting elaborated with complex cryptographic structures, called block chain or chain blocks. Therefore bit coin is called 'crypto currency' and is only the first of its kind. Users can manage their funds with digital wallets that have both a public key (equivalent to a bank account number) and a private key (equivalent to the bank account password). With both, it is possible to carry out financial transactions from anywhere in the world and at all times, so that, in addition, a currency that does not have territorial ties, as the fiat money does, results. In the same way as any other currency, it is possible to exchange it for local money, for which there are exchange houses or platforms like LocalBitcoins, which work all over the world.
It is abbreviated as BTC, and while the entire platform is written with a capital B, 'bit coin' in lowercase refers only to units of the currency.