There are many different aspects to skype.
Once you have downloaded it and created a username you can:
1. Use it to make free 'telephone' calls to other Skype users using a microphone; a headphone/mic combo or a dedicated usb skype phone; or a more expensive skype phone connected to your router
2. You can use it for instant messaging (like MSN)
3. You can buy a SkypeIn number so people without Skype can call you and pay normal landline charges
4. You can pay buy phone credit and call landlines and mobile phones
5. You can register your mobile phone and send texts to other mobiles via Skype for 5p.
What do you want to use it for? Have you downloaded it?