Both creams and lotions are emulsions having a water and oil phase. The salient difference is the thickness.
Lotions are a low-viscosity emulsions. Most lotions are o/w emulsions but w/o lotions also are formulated. Lotions are designed to be applied without heavy rubbing. Since they have a low viscosity, they usually are easily rubbed onto the skin.
Creams are much higher viscosity materials. They generally are semi-solid emulsions. Like lotions, creams can be either oil-in-water or water-in-oil emulsions. Oil-in-water creams are more comfortable and cosmetically acceptable as they are less greasy and more easily washed off using water. Water-in-oil creams are more difficult to handle but are more moisturizing as they provide an oily barrier that reduces water loss from the skin.