Just installled Ubuntu. But why can't I play video streams like those from Youtube?
Youtube and other popular sites providing video streams use Adobe Flash Player. The format is proprietary, thus, it is not part of the default installation of Ubuntu.
There is hope, though. Here is what you must do. Make sure you do not have any web browser open in your desktop. There are two ways to install the Adobe Flash plugin.
The first option is to download the package from this site: http://get.adobe.com/flashplayer/. And then open the file and follow instructions in installing it. It's very easy to install it.
The second option is to open a terminal (from menu Applications->Accessories->Terminal or by pressing Ctrl-F2 and entering 'gnome-terminal' command (without the quotes). With the terminal open, enter the following command:
sudo apt-get install flashplugin-nonfree
Just press 'Y' to confirm the installation.