This type of social communication takes place in real time in a similar way to a telephone call. When you are using instant messaging software you can reply instantly to a message. The person you reply to will read your new message and will be able to reply to it instantaneously if they wish. This is different to blogs and personal sites where communication can take place at any time without both people being online at the same time.
You can set up your instant communication software so that you can use a microphone to talk into and a web camera (webcam) to video you talking. The person at the other end can do the same and you will be able to see and hear each other while you have your talk.
Some messaging software allows a lot of people to use their web cams and microphone to have a conversation together. The software will let you see small windows with their faces in them. This is known as video conferencing.