Ethereum addresses are composed of the prefix "0x", a common identifier for hexadecimal, concatenated with the rightmost 20 bytes of the Keccak-256 hash (big endian) of the ECDSA public key. In hexadecimal, 2 digits represents a byte, meaning addresses contain 40 hexadecimal digits. One example is 0xb794F5eA0ba39494cE839613fffBA74279579268, the Poloniex ColdWallet. Contract addresses are in the same format, however they are determined by sender and creation transaction nonce. User accounts are indistinguishable from contract accounts given only an address for each and no blockchain data. Any valid Keccak-256 hash put into the described format is valid, even if it does not correspond to an account with a private key or a contract. This is unlike Bitcoin, which uses base58check to ensure that addresses are properly typed.
What miners are doing with those huge computers and dozens of cooling fans is guessing at the target hash. Miners make these guesses by randomly generating as many "nonces" as possible, as fast as possible. A nonce is short for "number only used once," and the nonce is the key to generating these 64-bit hexadecimal numbers I keep talking about. In Bitcoin mining, a nonce is 32 bits in size--much smaller than the hash, which is 256 bits. The first miner whose nonce generates a hash that is less than or equal to the target hash is awarded credit for completing that block, and is awarded the spoils of 12.5 BTC.
It is possible that the Ethereum Platform will not be used by a large number of external businesses, individuals, and other organizations and that there will be limited public interest in the creation and development of distributed applications. Such a lack of interest could impact the development of the Ethereum Platform and potential uses of ETH. It cannot predict the success of its own development efforts or the efforts of other third parties.