Bitcoin Private key is a 256-bit number that could be encoded in different formats according to the wallet in use. These formats include sets of randomly generated numbers and characters that variate in number of digits.
All these formats represent the same Private Key even though they look very different, but any format can be easily converted to another format.
Private Key takes the central role in ownership and control of your wallet and bitcoins or other digital currencies in it. Only with the Private Key, that corresponds your Public Key (Bitcoin address), you can use your funds in the bitcoin address as inputs for a valid transaction to another person.
The Private Key must be kept safe at all times because revealing it to third parties is equivalent to giving your funds away to the first stranger who wants to take them. Private keys should be backed up in a secure place because if you lose your Private key, it cannot be recovered in any way and your funds will be out of reach.