When Alice sends BTC to Bob, Alice creates a transaction, which is sent to the bitcoin network. However this transaction now needs to be validated and added to a block of the blockchain. The blockchain is a chain of blocks, and a new block is added every ~10min. We will explain how miners create new blocks, why it takes roughly 10 minutes to mine a new block, and why validation of a transaction takes on average 60 minutes if everything goes well (=6*10 minutes).
Miners are currently rewarded by 12.5 BTC by block (it used to be 25 BTC/block, now is 12.5 BTC/block until 2020, then afterwards 6.25 BTC/block. This halving continues until 2110–40, when 21 million bitcoins will have been mined.) A block consists of 2 things: - an easy part: a list of transactions since the last block was found, this is to validate the transactions. - a difficult part: a code that proves the identity of the miner that mined it (so that the network can award him the 12.5 BTC). Note that if a miner proposes a new block but that the block is found by other miners to contain invalid transaction, the miner will lose his reward.
How these 2 parts are linked: Given that validating the transactions is much easier than generating the code that validates his block, there is a strong incentive to only propose blocks with valid transactions.
Now we will explain why miners need a lot of computing power, this is because on top of validating transactions, mining also ensures supply of bitcoins at defined rate every 10min, and there is competition to get them! The rate used to be 25BTC every 10min, now it is 12.5 bitcoins every ten minutes until mid 2020, and then afterwards 6.25 bitcoins per block for 4 years until next halving. In order to achieve that defined supply rate, the network is constantly adjusting the difficulty of mining so that a new block is roughly found every 10 minutes.
A transaction is considered as finally confirmed after it is added to a block and ~5 other blocks have been validated. This means 6*10 minutes = 1 hour.
Now the problem is that there is since origin a limitation in the size of each block: 1Mb. This corresponds to ~ only 3 transactions by second. If the transaction rate goes significantly higher, it will create further delays in the validation process. This is the reason of the bitcoin forks.