Ceramides are lipids found naturally in the outer layer of our skin (=epidermis).  As a result, ceramides are very important molecules for healthy skin barrier function, which in turn can cause skin sensitivity and other more serious skin conditions and diseases if it becomes damaged or unbalanced. For example, one of the most common reasons for sensitive skin (but not the only) is a damaged epidermis. For more details check out: What is sensitive skin? What are the causes and what can we do?
It’s very easy to conclude then that ceramides are quite important compounds for our skin. However, is there any scientific evidence to support that and are they worth including in your skincare routine?
The term ceramide(s), abbreviated as CER(s), refers to a family of molecules (not just one compound – 12 classes identified so far [2, 3] and about 340 species )  and are therefore, many different ones that can be beneficial. However, this also means that your cosmetics might contain CERs or CER precursor compounds (=phytosphingosine and sphingosine) but not call them CERs in the ingredient list. You might also find them in their new and old names where: Ceramide 2 was replaced by two names,
Ceramide NS (limited to sphingosine-based ceramides) and Ceramide NG (limited to sphinganine-based ceramides); Ceramide 3 was replaced by Ceramide NP; Ceramide 4 and Ceramide 5 were both replaced by Ceramide AS; and Ceramide 6 II was replaced by Ceramide AP.  Ceramides are in general safe and non-irritating. 
Ceramides are one the most common lipids in our skin, 50% by mass  and amongst other functions they are also essential for preventing water loss.  As a result, CERs in cosmetics can be effective not only at replenishing the skin barrier function but also increasing hydration.  Ceramides are also potentially important lipids in cell death and proliferation. 
All these applications will in turn result in younger, more plump and firm looking skin with reduced lines and irritation. CERs can also be beneficial in haircare giving elastic, moisturised, smooth and shiny hair!
Ceramides can be extracted from animal and plant sources but can also be prepared synthetically. The latter can be preferred as synthetic processes have a lower risk of bacteria and other compound contamination.
If you think of our outer skin layer being our shield from the world, then CERs being an important building block of it will surely be very important. In fact, a study conducted in mice showed that a specific CER (CERSyn3) was essential for survival and mice that were deficient in it died shortly after birth! [6, 8] CER deficient skin was also more susceptible to other conditions such as yeast infections from Candida albicans [6, 8], Gaucher disease, psoriasis and atopic dermatitis.  It is not surprising then that CERs can be used as therapeutic agents to improve certain skin conditions and diseases, especially atopic dermatitis. Ceramides are also employed in the reduction and control of eczema. 
The cosmetic industry has also developed some compounds called pseudo-CERs, which are basically CERs with a missing or modified part. Some pseudo-CERs have also been showed to be beneficial for the skin and have very similar effects to CERS, including improvements in the skin barrier and moisturisation. [6, 10, 11] However, pseudo CERs are currently most commonly found in the Asian market. 
However, it’s not just about having the right amount of CERs. Ceramides are in general large molecules that can vary anywhere from 33-40 carbon atoms and the balance between them can be equally important.  Longer chain CERs provide a better water barrier than the shorter ones which means that conditions such as dry skin could arise due to having too many short chained and not enough long chained ones. 
Unfortunately, CERs can also be (but not always ) one of the many compounds that our skin looses over time during the ageing process, which might also explain partially at least, why some people report their skin becoming drier as they age. This makes them an even more important ingredient to include in your skincare routine, no matter of your age.
Although cereamides have received a lot more attention as topical treatements, there are some research studies that suggest they could be even more beneficial (~25%) when combined with oral supplementation. [14, 15] However, there aren’t enough studies on oral supplementation to suggest that you should invest your money in ceramide pills just yet.
If you are wondering how come something as important as ceramides only relatively recently started receiving attention by the cosmetic industry, the answer is very simple. Up until ~40 years ago we didn’t really understand how our skin’s outer most barrier works and with the progression of science and technology we are only now slowly understanding more about what keeps our skin healthy.  Sadly, there are still not as many research studies on ceramides as there should be considering how many of them there are and how important they can be, perhaps due to the fact that skin is a non-life-threatening research subject (although in some cases it can be) which in turn doesn’t receive funding as easily as actual diseases.  Nevertheless, there is evidence to show that ceramides are very important skin replenishing compounds and are worth including in your skincare routine.
As A. W. Johnson states in his research paper published in Dermatologic Therapy, there are four essential components to fundamental skincare: cleansing with mild surfactants and lipid depositions, protecting from UV damage, hydrating with humectants and replenishing the skin with skin lipids, one of which are ceramides.