Skype has several online statuses. Thanks to them your Skype contacts will always know if they will be able to reach you or if you are away for the moment. Some older statuses are removed in newer versions of skype, others have a different picture (this is why on most of them below you can see two pictures).
Users with this status are not signed in. They could have quit Skype or even turned off their computer. Therefore at the moment they cannot receive your calls, messages or files.
If a user is Online, this means that everything is normal, the user is signed in and ready to receive calls, messages or files.
This Skype status is not available in all versions of the program. It means that the user is ready to receive calls from everybody. Settings blocking people outside his contact list are ignored.
The user is signed in, but away at the moment. Keep in mind that depending on the settings, Skype can automatically switch you to this status (if for example you do not move the mouse or use at all the computer for 5 minutes).
The Not Available status is also an old one, and therefore missing in some versions. To be Not Available is almost like being Away, but this time you are gone for a longer period of time. By default Skype switches to this status after 20 minutes from your last action on the computer.
Do Not Disturb
This is a pretty nifty status. Do Not Disturb blocks pop up windows that show when someone calls you, sends you a file or a messages you. There will still be an indication for these events, but it will not interfere with your movie or work for example.
This is even cooler. If you are Invisible, you can see everybody, talk to them, chat with them and so on, but they all will see you as Offline. So they will never bother you with anything :).
This Skype online status appears when someone forwards their calls to a mobile phone or a telephone. Keep in mind that calling to anything different from a Skype client is not free and you need Skype credits to do it.
The question mark status
This is something similar to a status. It means that the user has not added you to his contact list. This status is not used in all Skype versions.
How to change your online status?
In order to change your online status, you first need to open Skype and sign in with your Skype name and password. Now right click on the Skype icon located in the system tray area (bottom right, near the system clock), click Change Status (or Change Online Status) and choose one of them.