These terms are used in different and sometimes overlapping ways by different journals. For example, a letter can mean anything from a peer-reviewed, high-impact brief communication (see Nature) to a letter to the editor. Even the letter to the editor category ranges from a simple, non-peer-reviewed paragraph to a peer-reviewed publication with data, even figures. In short, the answer to your question is journal- or publisher-specific and can be found by reading the authors' guides for your journals of interest.