Mining is the process of adding transaction records to Bitcoin's public ledger of past transactions. This ledger of past transactions is called the block chain as it is a chain of blocks. The block chain serves to confirm transactions to the rest of the network as having taken place. Bitcoin nodes use the block chain to distinguish legitimate Bitcoin transactions from attempts to re-spend coins that have already been spent elsewhere.
Where do bitcoins come from?
While traditional money is created through (central) banks, bitcoins are “mined” by Bitcoin miners: network participants that perform extra tasks. Specifically, they chronologically order transactions by including them in the Bitcoin blocks they find. This prevents a user from spending the same bitcoin twice; it solves the “double spend” problem.
Skipping over the technical details, finding a block most closely resembles a type of network lottery. For each attempt to try and find a new block, which is basically a random guess for a lucky number, a miner has to spend a tiny amount of energy. Most of the attempts fail and a miner will have wasted that energy. Only once about every ten minutes will a miner somewhere succeed and thus add a new block to the blockchain.
Below this video explain how bitcoin mining process
This Slide shows about bitcoin mining process( Click on the picture to stop)
