The job of miners in a cryptocurrency-network is to confirm transactions by solving the blockchains. The more transactions made, the bigger the blockchains and that is why miners need to upgrade their equipment every now and then. After solving a blockchain a miner is rewarded with a number of tokens or just a fraction, it depends on the value of the cryptocurrency that he is mining and the difficulty of the blockchain.
Many users forgot one of the most important features of Bitcoin—controlling your own money—and left more than 800,000 bitcoins in Gox accounts. In February 2014, Gox halted withdrawals and customers were unable to withdrawal their funds. The company’s CEO claimed that the majority of bitcoins were lost due to a bug in the Bitcoin software. Customers still have not received any of their funds from Gox accounts.
Double spending means, as the name suggests, that a Bitcoin user is illicitly spending the same money twice. With physical currency, this isn't an issue: Once you hand someone a greenback $20 bill to buy a bottle of vodka, you no longer have it, so there's no danger you could use that same $20 to buy lotto tickets next door. With digital currency, however, as the Investopedia dictionary explains, "there is a risk that the holder could make a copy of the digital token and send it to a merchant or another party while retaining the original."
Bitcoins are mined with powerful computer hardware and software. A maximum of 21 million Bitcoin will be available, after which no further bitcoins will be produced. The algorithm which governs the production of Bitcoin limits the quantity that will be produced, and the rate at which they will be produced. It is a finite commodity – there is a fixed amount, and that ensures that greater demand will always prop up the price. In this way, it is similar to other finite commodities such as crude oil, silver, or gold.