There are several applications that are capable of sending a video signal to another computer over the Internet.
For example, iChat from Apple, Yahoo Messenger, and SKYPE all allow video chats.
In iChat, if you have a web cam, and all modern Macs have one, you sign up for AIM or get a ME account with Apple. iChat uses the AIM network. After you log in, add your friends to your 'Buddy List.' When your friend goes online, you will see them listed in your buddy list. If they have a camera, and it is turned on, you will see a litle green camera next to their name. Double click on the camera and iChat will send a request to video chat. When your friend accepts the request, a window will open showing your friend's video feed. Within that window will be a 'picture-in-picture' showing what your friend is viewing of your video feed.
In SKYPE, when you connect to your friend, you can enable video from your side. Go to 'Preferences.' Click on the Video Icon, the 7th icon on the top of Preferences which looks like a blue dot with a camera icon inside. Check the box for 'Enable Skype Video.' This will start your video feed. Don't forget to turn it off when you don't want friends to see you. You can select options in the Skype preferences.
In Yahoo Messenger, there is a tiny icon on the bottom of the buddy list that looks like a TV set with a dot in the middle. Whe nyou click that, it will open a window showing your video feed, and send it out to the Internet. In Preferences, you can set the permissions about who can see your video feed: 1. The must ask for permission, 2. Everyone can see you, or 3. Allow certain people to see you.
In all cases you need a good Internet connection and a web camera which you might need to set up in some way depending on your particular system. Like any photography, good lighting is also beneficial.