@SlatternIsMyMiddleName - very good question re moisturizing and hydrating, so my explanation will be a little wordy. (Hope this helps.)
Moisturizers contain ingredients called emollients (fats and oils), humectants (glycerine and Hyaluronic Acid - also hydrating ingredients, but their purpose in moisturiser is to increase the efficacy of the water used as a solvent/emulsifier and make it available to the skin cells as well), Oils (Squalane, Nut Oils, etc) or Occlusives (Petrolatum, Parafin, Mineral Oil.) These are all pretty much designed to provide a protective layer on top of the skin to soothe dry skin, prevent chapping, provide a smoother surface for some makeups (depending on skin type and makeups) and the humectant ingredients can also penetrate the surface, cosmeceutical ingredients like ceramides, etc, but probably not as effectively as a serum would. (*Although more modern gel creme formulations definitely MUCH more effective than standard crèmes for this purpose, depending on skin type, these formulations absolutely have a place.)
Hydration is the most important (IMO) step. (Especially when heating and airconditioning is used.) Hydrators most used these days are Hyaluronic Acid and Glycerine. The most effective HA formulations have what they call “multi-molecular weight” HA, which means that it carries gets through to different levels of the skin and attracts water to those areas. It is believed that these products can also carry cosmeceutical ingredients with them more effectively also.
HA is the most amazing ingredient, but must be used correctly or it can actually CAUSE dehydration rather than help it. It must be applied to damp skin, and have some kind of occlusive layer applied afterwards or it can actually move back out of the area it has been applied to, taking water from the cells with it.
Moisturizing is important to stop further dehydration. This is an area where I personally wouldn’t invest huge amounts.
Moisturizing ingredients are almost always cheap to produce and do the same thing. You just have to find a formulation that suits you. Most people have skin that is dehydrated. Very few have clinically “dry” skin. Slapping thick creams on the surface of the skin won’t address what’s going on underneath it, and can actually cause irritation and clogging if the wrong ingredients are used and the skin is desperate for water.
If I am totally honest, I don’t use a separate moisturizer during the day unless my face is extremely dry - and that doesn’t happen anymore. I use my HA lotion (it’s a serum, but they call it a lotion) , spot treat some discoloration with vitamin C serum, then follow with sunscreen, which serves as a moisturiser/occlusive. I use a light gel creme at night because we have aircon on already.
I also use cheap, scented body lotion from drugstore as my skin isn’t sensitive, but in winter I always bump up to LRP Atoderm or equivalent to stop the dry skin itchies. (Can use on face if required then!)
Hope that helps!