Before today, if you wanted to Direct Message another user on Twitter, you and the other user had to following each other. It's one of the classic Twitter "rules" that has been around for years, but today Twitter is finally giving you the option to change all of that. From now on, you can receive a DM from any one that follows you; and you, in turn, and message anyone who you follow. In order to enable the option, all a user has to do is go into their main settings and tick the option that allows it. Like so:
Yet, that means you could potentially get a ton of messages flooding your inbox (but as long as you don't turn on the option, you're good), but at least you get the choice of picking what you'd rather have. If you're a person who gets a lot of @ replies from users asking you to follow them back so they can DM you a message with "important information" or the like, this new feature will practically end those types of messages. (You get this a lot if you're a writer. Cough.) No telling if this will result in more spam, though, but if it does, you can just revert back to the old school way.