Under Linux, there are several tools available, but for those of us who like Linux then the chances are somewhat a bit limited. If you are a KDE user or a Gnome lover who's willing to accept only those programs that are written in GTK, then record MyDesktop is one of the best, free apps that you can use to easily record your desktop sessions in Linux.
They have written binding for it using both QT and GTK+ environments and comes with an easy to use GUI as well. The only drawback is that it only support ogg Theora file format (a filly GNU compliant codec). So if you want it to be avi or something else, then you'll have to manually convert it. But most of the never multimedia players should be able to play then nonetheless.
If you use Ubuntu, then you can easily install it using the software center. Open the "software center" and search for "recordmydesktop" within the GUI, which should bring you something similar to the below screenshot, and then choose the "gtk-recordMyDesktop". Then click "install" for installing it automatically.
As said, it has a simple GUI and within it you can change hue, brightness, video/sound quality with ease as well. If you want more advanced settings, then click on the "advanced" button in which you can choose various other settings such as file compression (if checked, the output file will be reduced as best as it can, but the encoding may take a bit longer or more of your PC resources to be precise), etc.
You can choose to record your entire desktop (which is the default option) or can make certain portion of the screen only to be recorded. Once loaded you can access it via the notification area using the "Red circle" as well. Although you can install it using an internet connection since it is support by most of the distributions, sitll you may wanna manually download it in some cases.