Twitter has got to be one of the greatest Social Networks that has gone up on the internet. Its extremely simple.
Its very simple to send a message. There is a big box on your home page when you go to twitter.com/home . You type in there up to 140 characters, and everyone who follows you can see what you are posting. Also you can put @username of someone on twitter, and that will allow them to directly see the Tweets, the @ also has other uses.
Basically, at every users personal page there is a giant button that allows you to follow that user. After that, the Tweets get displayed on your home page
For those more private messages that need to be sent directly to the user, there are direct messages that are alot like replies, except they can only be viewed by the user it was sent to, and you can only direct message people who are following you AND you are following them.