What is Saccharomyces? A very brief mycology lesson.

Saccharomyces is a fungus.
Not the sort of fungus you see when discover ancient sandwiches in your locker, or the sort of fungus you may slice and add to a stir fry.
More specifically, Saccharomyces is a genus of fungi (kingdom: Fungi) that includes many, many, species of unicellular yeast.
If you are equipped with a brain, you'll know that you cannot have beer without yeast. What you may not know, is that almost all of the brewing yeasts used to ferment the beer you're drinking will come into the genus Saccharomyces. Scientists out there might know all about this yeast, if not, they might have picked up on the name 'Saccharomyces' and seen that it is a sugar consuming fungus (saccharo - sugar, myces - fungus). If you didn't pick up on this, then you're not a good scientist. Go study commerce.

So who cares?
If you enjoy learning and enjoy drinking beer, then you will care. I was utterly thrilled when I started learning about yeast, then fermentation, then glycolysis, then the citric acid cycle, and the organic chemistry behind it all.
Saccharomyces cerevisiae is a species of yeast - a species that all craft beer drinkers will be well acquainted with (whether they know it or not). Microbiologists are also rather fond of it, as it's an ideal organism to experiment on. This species is commonly known as Ale Yeast, or perhaps Baker's Yeast, but the former is far more important (cerevisiae meaning "of beer"!)...
Some would think it's enough to go from a Kingdom of life through phylum, class, order, family, genus and finally to a species, but brewers (among others) take it much further - culturing species and subspecies of S. cerevisiae for years to yield different strains - they'll even give them warm, loving names such as '1028' (a London Ale strain).

S. cerevisiae love the little bubbles of carbon dioxide that accumulate in the fermenting beer, and so they hitch a ride on each one up to the surface, hence, becoming a top-fermenting (ale) yeast - brewers call this floating, foamy yeast 'head' a krausen.
S. cerevisiae's cousin, Saccharomyces pastorianus, tends to stick around the bottom of the fermenting vessel. Different strains of this fellow are used to brew lagers.
S. pastorianus used to have a different name - one hint: "Probably the Best Beer in the World"... Saccharomyces carlsbergensis is the other name for our lager friend, and was given that name by Emil Christian Hansen, who worked for Carlsberg back in the day (1883).
A lager like Carslberg (with it's far-too-confident-assumption tagline) is absolutely nothing compared to a well brewed ale. Despite 'well brewed' excluding Carlsberg from the competition, it's yeast doesn't do it any favours, either.
Ale yeasts tend to produce more of those nice fruity esters, a higher alcohol volume and they ferment at higher temperatures unlike lager yeasts. Also, 'lagering' (storing) is normally required with lager beers, as their yeasts can make some pretty nasty flavours that take time to disappear (the yeast reabsorbs them).
Don't get me wrong, I enjoy a cold, fizzy lager once in a while just to quench the thirst... Truth be told, I 'enjoy' them because they're normally cheaper than preferred ales...
The versatility of ale yeast is highly appreciated by some brewers, and so they use it in their lager-styled beers (although they may not advertise this). Funnily enough, many mass producers of lager will label their beers as ales *cough*TUI*cough*SPEIGHTS*cough*!!!
I digress.

So hopefully you've not become too bored or confused, and learnt something after reading this.
At least, you'll appreciate my fondness of Saccharomyces. I love everything about it, how it works, how it smells (I get strange looks when I stick my nose into petri dishes of cerevisiae in the lab), I even love it's laxative effects when you eat it alive (...)
You should love it, too - after all, it gives us beer.