When it comes to Bitcoin, a private key is a secret number that allows bitcoins to be spent. A private key is created by a software program designed specifically for this purpose and is typically stored in a file called a wallet.dat. A private key is mathematically related to all Bitcoin addresses generated for the wallet.
Because the private key is the “ticket” that allows someone to spend bitcoins, it is important that these are kept secure. Private keys can be kept on computer files, but are also often written down on paper.
The mathematical relationship between the private key and the public key (address) is such that the Bitcoin network can verify that the signature attached to a transaction using the public key is valid, but cannot derive the private key from the public key. This system of verification is what makes possible for Bitcoin to operate without any central authority; no one but the owner of a private key can send Bitcoins from that address.
A private key is simply a number between 1 and 2^256. While it’s technically possible to brute force an answer by trying every single number until you find one that works, the odds of doing so are about as good as your odds of winning the lottery twice in a row.
In other words, not good.