Oil & Water/Serum & Lotion

I started moisturizing my face my first year in college.  I'm not sure why; it just seemed to make sense, and it felt good on my face.  Now, after, ahem, several years of moisturizing, I've tried just about everything.

That is, until I started using natural oils.  It started with a great serum from Naturopathica.  Then I started blending my own serums.  It's amazing how great my face feels using these nourishing oils.  Who knew?

What I'm trying to understand is what the difference in effectiveness is between lotion (water & oil, basically) and serums (oils).  Does anyone know?  What's the benefit of a lotion?  A serum?

In the meantime, I'm going to stick to my serums (evening primrose, rosehip, pumpkin seed, avocado...ahhhh....)