Well, the real difference between cream and serum is that the serum is liquid and the cream is "creamy" - not liquid.
Serums can be characterized by textures are very different, from the most fatty (containing 100% fatty substances) to the emulsion and most buoyant, containing mostly water. And there are many intermediate levels - emulsions in all kinds of textures, enriched to more buoyant. Some contain vitamins, some other active substances, and some of them don't have any vitamin or any other unique ingredient to differentiate them from the cream. Serums are usually thinner and lighter than creams and can have a great variety of functions. The common denominator is that they are liquid serums and are applied directly to the skin after cleansing and/or toning and before your cream and sunscreen.
For example, if you are looking for a moisturizer you can choose between Lifting Cream and Lifting serum and you usually won't need both of them since they include a lot of the same active ingredients (Vitamin C, Vitamin E, Organic aloe vera and Hyaluronic Acid) and both act as a lifting moisturizers. The Lifting serum will boost the hydrating effects of your moisturizer cream, but in most cases you will need only one of them.
The most important factors in determining if you should select a Serum or Cream are your skin type, weather, and your living environment.
Cream-based skincare (such as Lifting cream) is most suitable for people with dryer skin since they lack moisture and are most prone to tightness and peeling. Reinforcing skincare with cream-based products will help restore sebum and moisture to the skin, thereby delaying the speed at which moisture escapes from the skin.
People with combination skin and those who live in humid environments can also use cream-based skincare during seasonal changes. You can reinforce your hydrating efforts by simply applying cream based products to areas prone to dryness and peeling. People who work in air-conditioned areas can also reinforce their skincare with cream-based products to protect skin from moisture loss.
Oily and/or acne prone skin are prone to oil and have slower renewal cycles. If you use cream-based products with high oil content, it can cause your pores to become clogged, thereby increasing occurrence of pimples or acne. For people with oil or acne prone skin, I would recommend using oil-free serums or lotions with lighter consistencies to lock in moisture (such as Lifting serum).
Since I live in a hot climate and have normal skin type (check out the post on Various Skin Types and Conditions) as a moisturizer in the summer I use only Lifting Serum, in the autumn and spring mostly Lifting Cream and only in the winter I use both of the products. The point is you will probably need only one of them most of the year as your moisturizer.
What is important is finding the serum and/or cream that will suit best for your skin needs.