Skype is an instant messenger and communications app. It is best known for its ability to connect to people all over the world. With it, you can call any Skype users for free and have real-time voice conversations with them. Also, if you put money on your account, you can call any phone line in the world from your computer. If you would rather type than talk, Skype has a good enough IM system, complete with emoticons and file attachments.
Video conferencing is a great feature of Skype. The newest versions allows you to chat with your contacts, see them and let them see you. You can even activate video chat in full-screen, where you see your contact in a big, full-screen image, and you see yourself in a small square at the bottom. The quality is very good, but it depends on the kind of hardware that you have and your speed connection. The standard iSight camera that ships with all Macs allows you to have great quality video chats out of the box.
The Mac and Windows versions are quite similar right now. However, it takes a couple of versions for Windows features to make it to the Mac counterpart. For example, full-screen video has made it to the Mac just recently.
The rest of the features are all available both for Mac and Windows: online number, Skype out, voicemail, etc.