The Comments feature is extremely useful in documenting all contact with your leads. Each comment is date and time stamped by user. For example, if you have a telephone conversation with your prospect, you can quickly type in some notes about the conversation for review at a later date.
Other examples of what you can use Comments for include copying the body of an email reply you may have received from the Lead. To add a comment from the Lead Detail screen, click "Add Comment" in the History Section on in the right column.