Dizziness is a term that is often used to describe two different symptoms: lightheadedness and vertigo. Light-headedness is a feeling like you might faint. Vertigo is a feeling that you are spinning or moving, or that the the world is spinning around you.
There are probably many symptoms for each, but some common ones are:
1) a lack of oxygen to the brain.
2) pressure changes or fluid on/in the inner ear.
3) low blood pressure.
Of course, then the task of finding the cause of whatever symptom fits the specific individual is a deeper matter entirely.