Hi,
If they are Using the Finder's Go Menu > Go to Server and entering in VNC://(IP Address) to get access you have the Access specified in System Preferences > Sharing > Screen Sharing.
This can be set to allow Specific People or get them to Ask you.
If they Login by this method as You they have the same Access you do. (Not only your Home Folder but any other File and folder you can access in normal navigation of your computer - includes System Folders and Mounted Drives)
If they Login as another User they get the access to your Account in a similar fashion.
You can set a Password in the Computer Settings that restricts the Control from their end (they can only see where you lead)
In iChat is is as if you logged in from a second computer. The person if they are Sharing your Screen (they get to see your Screen) they can move the mouse and Click anywhere you can go.
If you then Use Fast User Switching iChat will do whatever you have set in the iChat > Preferences > General Settings to do when you use Fast User Switching.
If this is to be Away then in theory they could be looking and messing with all your stuff while you are busy elsewhere.
(I say "in theory" as I have never actually tested this. I do know that iChat is not Logged Out. They should not be able to see where you go in the other Mac User though)
And yes, it was a totally different question 🙂
8:23 PM Wednesday; October 13, 2010
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"