WIF keys can be either 51 or 52 characters long. The ones which are 51 characters long are for uncompressed private keys and start with a "5". The ones which are 52 characters long are for compressed private keys and start with either a "K" or a "L". They are in Base58 and are case sensitive. The compressed keys include an extra 0x01 byte prior to the checksum, which tells your wallet software to generate the compressed public key rather than the full public key when imported.
Standard private keys are 64 characters long. They are in Base16 (hexadecimal) and are not case sensitive.
Further further correction: a private key is a number.
The number can be written in several different ways including binary (base-2), octal (base-8), decimal (base-10), hexadecimal (base-16), base-64, and WIF.
Of these methods, WIF is the most common in Bitcoin, hexadecimal is the most common in Ethereum, and base-64 is the most common most other security applications.
Also note that in hexadecimal, there is no difference between upper case A-F and lower case a-f. They both represent the same values. However, there is an extension of hexadecimal used by Ethereum that uses the case of the letters to validate the value of the private key.