Comments are notes left on a question or answer. Comments do not generate reputation, there's no revision history and when they are deleted they're gone for good. Comments are a way to clarify something that may have not been clear in your question or answer OR a way for another user to ask for clarification in your question or answer.
Please note that you can always comment on your own posts once you have been granted the 'remove new user status' privilege. However, commenting on others people's posts is what this privilege grants.