Icons and symbols on the screen
Icons and symbols may differ a little bit, depending on the distribution, your personal settings, and perhaps your system administrators personal preferences.
If you are using KDE or Gnome I think you can enable an option called 'Show tooltips' or something similar. If you do that, a small description of the object will appear keeping the mouse on the object for a few seconds. (I'm not 100% sure about this though, correct me if I'm wrong)