Thanks for reaching out to Optiverse!
With audiences you can manage who is going to see your experiments e.g. desktop vs. mobile users, Firefox vs. Chrome, etc. It is basically a way to filter out specific visitors.
Dimensions help you to segment your results. Optimizely provides default segments which can be used to filter your result e.g. for a particular browser type. However, you can also setup custom dimensions (= custom segments) and use them within the audience settings, as well.
I hope this helps!
EMEA | Amsterdam