As you can see from this Import Contacts wizard screenshot, Skype makes it as easy as possible for you to import your contact lists from Outlook, Entourage, etc. into Skype. The process is automatic once you start the import Wizard; on my Windows machine it took less than five minutes.
Why would you want to do this? Well, because every person that signs up for Skype on your contact list and uses it to call you (or vice versa) is getting a free phone call, anywhere in the world.
In order to start the Import Contacts wizard, navigate to Contacts, then Import Contacts while you're in Skype. The process should start immediately.