I worked for years at a big outdoor shop and this is some of the story that I gathered from the stores’ buyers and product reps:
Leather
The hide is taken from the animal and the hair/fur is removed and the skin is tanned to prevent it from rotting. It is dyed if a different colour is desired and otherwise chemically treated/finished to achieve greater function beyond its natural characteristics of strength, flexibility, breathability, and water resistance.
The hide fibres are most densely packed at the skin surface, the smooth side, and are less dense towards the inside flesh. The hide is normally too thick for many uses so it is split into layers.
The outmost layer of the hide is the “top grain” and has the most dense structure and is naturally the most water resistant and durable. Because any hide only has one top grain layer, it is considered the highest quality and is the most expensive.
If the leather uses most of the thickness of the hide and includes the top grain, it can be referred to as “full grain” leather and is spec’d according to thickness, for example “2.0mm Full grain Salpa(proprietary treatment) leather”. This is the kind of leather used in the classic Norwegian welted telemark boots.
Usually, the outermost skin is used “smooth side out”, having the almost membrane like, dense-ist part of the leather on the outside of the boot (again like classic tele boots) providing the most structurally water resistant part of the leather protecting the wearer. The downside to this is that this super-tight, less-porous smooth-skin-side-out is that it is prone to abrasion and scratch damage.
The inner split grain layers are made up of a less dense matrix and while more breathable, they are less durable and water resistant. This is “suede” and has a fuzzy appearance and feel. Because they are lower quality and often more than one split grain layer can come from the hide, this is the cheapest leather. Split grain leathers are thinner, like 0.9-1.5mm. Sometimes these cheapest and most porous leathers are PU (polyurethane) coated to give a smooth-out appearance and help waterproof the leather, but of course it reduces breathability and will wear off.
Full grain/top grain can also be used “rough side out” which might look like suede(a split grain) but because it includes that very dense top grain, but turned towards the inside, abrasion and scratches have a harder time getting deeper into the leather. While the outer surface of these boots are a little fuzzy and don’t repel water as readily, water doesn’t penetrate as quickly because the leather fibres are denser to the inside.
“Nubuck” (as I understood it) is a high quality top grain leather that is brushed to give it a little soft “nap” or fuzz for aesthetic purposes, while opening up the breathability slightly and adding surface area to hold a touch more repellancy chemicals. The brushing can also even-out blemishes in the leather that otherwise would be rejected for full grain/top grain applications.
Treatment
The original factory treatments are petro-chemicals, silicones, and synthetic waxes. The finish achieved in the manufacture of the leather is typically the best it can be for its intended use. Applying aftermarket treatments is sort of like trying to improve/replace the factory paint job on your car with a can of spray paint. However, even the new finish on leather boots do wear out, getting abraded, drying out, getting dirty and contaminated.
Silicone sprays make the least impact on the look and feel of the leather (but can still darken and de-fuzz the finish). The water and dirt repellant chemicals are carried by solvents that evaporate quickly, but also prevent the active ingredients form absorbing very deeply into the leather. Spray treatments have to be re-applied most often.
Synthetic “waxes” (eg. Nikwax, Zamberlan's Hydrobloc Cream, or Biwell Green mit Silicon) try to create both a chemical and physical barrier to dirt and moisture. Their very nature (thick at room temperature and colder, and chemically non-polar(H2O doesn't bond)) however, tends to prevent them from being absorbed very deeply into the leather as well - the positive is that breathability is not hindered completely. Some will suggest warming the boots first then working and melting the wax into the leather to maximize absorption and then letting the treatment cool and congeal. A final buff gives a nice shine and DWR type repellency.
Generally, for modern performance footwear, any grease or oil based treatments(Dubbin, Mink oil) are discouraged. While we know that oil and water don’t mix - that’s good -, the other properties of oils and grease weaken the structure of the leather: The oils do absorb more deeply but they can clog the porosity and breathability of the structure and excessively degrade and lubricate the leather fibres such that they can tear apart and rot. Living skin is continuously nourished by body oils when a creature is alive and new skin cells are regenerated as the old ones slough off - of course, not so with our leather boots. Oils and grease significantly darken and smoothen leather.
Oils and grease based treatments can be helpful to soften or break-in really thick, inflexible leathers and to help rejuvenate very dried out leathers.
Snow Seal is beeswax based and BiWell Red (supposedly, has the natural oil lanolin, from sheep’s wool) are somewhere in-between sythetic waxes and greases.
One note about breathability (water-vapour permeability) that a rep pointed out is that it is minimal through the boot materials, unless the footwear uses fabric mesh panels(ie. no waterproofness). Most of the moisture that doesn't get stuck in your socks or the boot lining, is really only vented out by the "chimney effect" of warm vapor blowing out the top opening of the boot..
I hope to be in Trondheim cheering the Swedish team on for the World Cup! 
