Bitcoin mining is a process by which Bitcoin transaction records are added to the blockchain. The blockchain is Bitcoin's public ledger of past transactions and is essentially a chain of blocks. The blockchain confirms to the whole network that transactions have taken place.
Bitcoin mining is a decentralized computational process that confirms transactions in a trustful manner when enough computational power is devoted to the block. That way, legitimate Bitcoin transactions are distinguished from attempts to re-spend already spent coins. The Bitcoin mining process also creates or issues new Bitcoins in each block. Bitcoin mining is, therefore, an integral and vital part of Bitcoin because it keeps the network secure, safe, and stable. In other words, without Bitcoin mining, there would be no Bitcoin exchanges and quite frankly the currency wouldn’t exist.
When Bitcoin initially started, the mining process was carried out with central processing units (CPUs) of regular desktop computers. However, as the popularity increased, graphic processing units (GPUs) replaced CPUs and became the dominant hardware for Bitcoin mining.
In time, newer hardware was designed specifically for Bitcoin mining. The hardware, known as Application-Specific Integrated Circuit (ASIC), was first released in 2013 and has since been improved upon to fit more into market demands. Bitcoin was originally designed in such a way that anyone with the required hardware (computer CPUs) could participate in the mining process. Doing that meant they could be rewarded for the service. However, as the hardware needed for mining became more complex (ASIC), mining became difficult to impossible for anyone unable to make the enormous investment (thousands of dollars) on the hardware. Furthermore, because the mining process requires a lot of electricity, powerful cooling units, and strong network connection, even fewer people were able to remain miners. Bitcoin mining is an intense computational process that becomes more difficult as more miners try to create new blocks in the blockchain. That's why it was initially easy at the start when fewer miners were using CPUs, and why only ASIC mining hardware can effectively mine it now.
Credit: Josh Wardini - firstname.lastname@example.org