One of the main draws of Bitcoin is the fact that it is decentralized; there is no governing body or state that controls the currency. This is unlike our conventional forms of currency, which are all run by centralized banks in their respective nation—The Federal Reserve in the United States, The Bank of Canada in Canada, the European Central Bank for the European Union, and so on.
With Bitcoin, there is no physical object to represent a bitcoin, nor is there a single entity that decides to create more Bitcoin. Instead, the concept of Bitcoin was created with a finite number of Bitcoins that will ever be made, and a public ledger that keeps track of every record. In total, there will be 21 million Bitcoin that can ever exist.
The blockchain is that public record and it is made possible through thousands of powerful computer systems across the planet that are maintaining it and recording transactions. At the time of writing this, there are over 10,000 different systems, known as nodes, operating to maintain the blockchain.
Because it operates on a transparent ledger that is based on peer-to-peer support, it is virtually impossible to fake a transaction on the blockchain. To do so would require an individual to have control of at least 51% of the 10,000+ nodes that are recording transactions.
You might be wondering why more than 10,000 computer systems would bother maintaining the ongoing transaction records of Bitcoin. Because, in exchange for that service, these systems are also mining Bitcoin.
Mining just refers to the act of facilitating and recording the transactions of the blockchain. In exchange for their processing power, those who do the mining are paid a fraction of a Bitcoin. As new transactions are made, these miners create new blocks (a group of transactions) on the blockchain.
The only way to put the 21 million Bitcoin possible into circulation is to continue mining and facilitating the blockchain. At the time of writing, there is still nearly 2.4 million Bitcoin left to be mined.
As more and more mining occurs, the amount of Bitcoin paid to miners is halved. It is estimated that the last Bitcoin will be mined in the year 2140.