DM Full Form is Direct Messages. Several websites let you send DM or private messages to anyone in your list. Though direct messages are similar to email, there are few distinctions. DM has only few features. It can be send only to people who also use the same website. Direct messages cannot be a replacement to emails. Websites like Twitter, Instagram, Facebook, etc. allow direct messaging and each of them have their own set of restrictions and features for DM.
DM is often used wherever emails are not an option. For instance, eBay sellers have their email address as private while they let people contact them through DM. In other words, direct messages enable communication between two people or parties without having to share their email address. Online stores and social networking sites are the two main kind of websites where direct messages are commonly used.