The icons do not actually mean anything inherently, but are merely graphical labels. There are a selection of icons that the game uses in order, which may or may not be appropriate for the skill you just learned (I once got the skull and crossbones for a healing spell). You can click on the 'change icon' button in the skill window to choose which icon is associated with a skill.
Hopefully there is enough variety that you can assign icons that are meaningful to the skills and will let you easily distinguish between them.
