Morse Code
Morse Code, created by Samuel Morse, was designed to transmit letters across telegrams. He wanted frequently used letters to have short codes and less frequently used letters to have longer codes to make transmission faster. It has since been used in many other situations. For a lot more information, visit Wikipedia.
When you encode messages, typically one will leave out punctuation. When decoding, only periods and hyphens will be decoded and the rest will be left as-is. This site uses International Morse Code with some additional enhancements. There's also a few tools to help you decode snippets you find by allowing you to reverse the input (flipping the text backwards) and swapping periods and hyphens.
One of the more famous places that utilizes Morse Code is the Kryptos statue. Check out photos of the sculpture. This sculpture has several messages that can be entered into this decoder. (this is sent as the prosign), , , (the extra E at the end of VIRTUALLY doesn't have a large enough gap to make it a separate word), , , and . If this thing interests you, there's an expanded page with more info.
Here are all of the letters I understand, followed by all of the prosigns and more complicated meanings.