This tool can be used to encode and decode messages using a Vigenere cipher.
The Viginere cipher was an early Polyalphabetic substitution cipher, where the user cycles through multiple cipher alphabets in an effort to defeat frequency analysis. This system was able to side-step the flaws of earlier monoalphabetic substitution ciphers for several hundred years, until Charles Babbage (and others) identified a key weakness in the cipher during the nineteen century. To use this tool, present your message and a multi-letter key (longer is better) and it will encrypt the message.
The weakness of a "raw" vigenere cipher is it's tendancy to repeat letter patterns at specific intervals as a result of cycling through the cipher alphabet with similar words / underlying trigrams. This is known as the "index of coincidence"; once you examine the distance between repetitions (specifically, the factors of this distance), you can start making educated guesses about the length of the cipher key. This allows you to split the message into the constituent alphabets (which are merely caesar shifts). These simple sub-alphabets can be attacked with frequency analysis and brute force tests.