Hashing is when a variable length message is transformed into a unique fixed-length alphanumeric string. There are various hashing algorithms available, such as MD5 and SHA1. Hashes are one-way and non-invertible, unlike symmetric encryption functions, such as AES, which can recover the original message if you have the key. Because hashes cannot be reversed, most of them are cracked by brute force. Crackers will build power-sucking rigs with several GPUs to hash every possible character combination until they find a hash that matches. They will also generate rainbow tables or files containing all of the hash outputs generated for quick lookup.
Salting your hashes is important for this reason. Salting is the process of adding a random string to the end of the password, provided by a user, to add more randomness or entropy. Consider an application that stores user login...