It's called TeamViewer and it's the best remote assistance software for non-commercial use.You can get it at http://www.teamviewer.com/
You can either install it or just run it for one session.It's small and compact and overall a great piece of software.It's also good for transferring multiple large files from one computer to another.

The interface is pretty self explanatory.You have your ID and Password which people can use to connect to your computer on the left and the option to connect to another computer on the right simply by putting in the ID and Password from the other computer.
There's a Meeting option previously called Presentation where you can invite people to just look at what you're doing on your desktop without having any control unlike the remote assistance sessions.There's also live video , VOIP (Voice Over IP) , Conference Calls , Chat , File box and a Whiteboard function where you can point to stuff on your dektop via a paint brush , pencil , rectangle tool there's even speech balloons you can add for everyone to see and many more.And all that for free.
And best of all it's cross-platform that means you can download the Linux version and someone on Windows will have no problem connecting to your computer.