Most of the crypto currencies work on the idea of decentralization i.e. there is no centralized server. Instead they depend on multiple computers owned by different individuals to run the server and maintain the network.
Because these individuals do not hold any direct stake in the technology, a reward system know as crypto currency is generated. These types of coins are those that are given as an incentive to the miner who manages to solve a complex algorithm and, in turn, validates the transaction and adds the newly created block into the blockchain. All these miners helping the system, are rewarded based on their contribution.
In the earliest days of Bitcoin, mining was done with CPUs from normal desktop computers. Graphics cards, or graphics processing units (GPUs), are more effective at mining than CPUs and as Bitcoin gained popularity, GPUs became dominant. Eventually, hardware known as an ASIC, which stands for Application-Specific Integrated Circuit, was designed specifically for mining bitcoin. The first ones were released in 2013 and have been improved upon since, with more efficient designs coming to market. Mining is competitive and today can only be done profitably with the latest ASICs. When using CPUs, GPUs, or even the older ASICs, the cost of energy consumption is greater than the revenue generated.
According to coinmarketcap.com, examples of Top Ten mineable crypto currencies are Bitcoin/Bitcoin Cash, Ethereum, Dash, Litecoin, Monero, Ethereum Classic, BitConnect, Zcash, and Steem.
The total number of mined Bitcoins so far is 16.5 million, according to Steemit and 4.5 million coins are yet to be mined. Mining crypto currencies such as Bitcoin is becoming harder every day as the number of block rewards continues to reduce. In the future, small scale miners will be at a disadvantage as the costs of mining will be high. Large scale miners will benefit from economies of scale. Mining difficulty and a reduction in block rewards will increase the prices of minable crypto currencies.