What is Proof of Work

Proof of work is a core component in the verification and generation process (mining) of Bitcoin and other alternative cryptocurrencies.

It functions as a process to show that work or effort has been expended to achieve a desired set of data. The amount of work or effort used to create the proof of work is defined by the difficulty, which is more or less an arbitrary condition determined automatically and mathematically.

For the case of Bitcoins and similar altcoins the desired data is the final block that is generated (containing most recent transactions as well as all previous transactions) and meets a value that matches with an arbitrary condition. The final block is a demonstration of energy and effort to validate and verify a set of transactions and all transactions that proceed it.

Both Bitcoin and the second popular cryptocurrency, Litecoin, uses the Hash Cash function as its proof of work. Producing this proof of work involves finding valid solutions to complex mathematical problems through a series of random trial and error (brute-force) attempts using computational power.

While proof of work seems like an unnecessarily arduous task, it creates an environment where users go into an consensus agreement of rules to enter a contest to solve for the next block to receive the hard-earned reward. In addition to this, the connected nature of the block chain and the difficult nature of producing a proof of work makes the integrity of the block chain very difficult to compromise. Any attempts at foul-play would require all the work already completed for previous transaction to be done again starting from the very first block.

source: http://www.cryptothinktank.com/education/proof-work/

What is Proof of Work