Base58 is a method of displaying alphanumerical characters converted from binary data. Base64 is very common on the internet but Base58 uses six fewer characters. Bitcoin address use Base58.
Base58 removes easy to confuse characters such as a lower case "l" which could look very similar to the number "1" or the capital letter "I" to the human eye. Base58 includes the following characters: 123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz