How about the seasoned performers on here explain the following for the less seasoned:

Why 2.35cm?

Why 2.1cm?

Why high tyre pressures (over 35psi)?

Why lower tyre pressures(under 25 psi)?

Why tubeless?

Why reverse the tyre direction?

Preferred tyre thickness for winter/muddy conditions?

Preferred tyre thickness for dry/summer conditions?

Prefered method of puncture protection?

Ok, my tuppence worth (breifly)....

Why 2.35cm?

Generally for rockier riding, usuallly offer more grip and comfort and should give you a bit more confidence to attack technical stuff.

Why 2.1cm?

Why not? I would only use them for less technical stuff, eg forest and moorland rides, places with lots of fire road etc. But are you talking fast 2.1s (eg Racing Ralphs) or slow 2.1s (eg Trailrakers)?

Why high tyre pressures (over 35psi)?

I only use higher pressures for less technical trails and racing

Why lower tyre pressures(under 25 psi)?

For more comfort on my hardtail, though tbh I wouldn't drop below 25psi unless I was running UST as there is too much potential for pinch flats.

Why tubeless?

No punctures which is great when night riding!(there are many other benefits too but I can't be bothered to type them all out)

Why reverse the tyre direction?

I find that Nobby Nics offer more grip when direction is reversed (on the front only)

Preferred tyre thickness for winter/muddy conditions?

Some will say thinner tyres to cut through the mud, other people (freaks) use big fat tyres at low pressures.

Preferred tyre thickness for dry/summer conditions?

Same as winter for me lately, though if it's really dry I'll stick on some Racing Ralphs, still using 2.3s though as most of my riding is fairly rocky.

Prefered method of puncture protection?

UST.

Why 2.35cm? Because its slimmer than a 2.1 and fits in your frame.

Why 2.1cm? Because it can have more volume than a 2.35

Why high tyre pressures (over 35psi)? For fast rolling

Why lower tyre pressures(under 25 psi)? To get pinch flats if running tubes

Why tubeless? Because you run at less than 25

Why reverse the tyre direction? So as you can read the sidewall writing

Preferred tyre thickness for winter/muddy conditions? Eh

Preferred tyre thickness for dry/summer conditions? Eh

Preferred method of puncture protection? Pressure over 25 PSI

Well cheers for that.......now anyone who comes on asking silly questions about tyres can be shown this. 

Tbh, for the beginner it is a bit of a minefield so it's good to get all the info on one page.

http://www.bikemagic.com/news/article/mps/uan/3675

The Maxxis Advantage is a tyre I and a lot of the people I cycle with are very big fans of. Works great in just about everything but full on deep mud, great grip, especially in the wet and a fast roller. They work well on the road as well as both built trails and natural forest surfaces, which covers approximately where I cycle. Also pretty cheap and wear slowly... If you're too lazy to change tyres for the conditions any given day these won't catch you out.

What tyre you like depends a lot on where you cycle. Where I am (West of Ireland) there's a lot of limestone, a rock that gets greasy when damp as well as plenty of rooty stuff, soft ground and plenty of rain, so it works well for us. In the east of Ireland there's more granite and sandstone, so the high roller (Designed for fast rolling hardpack and rocky surfaces IIRC) is a more popular tyre there.

Reversing the tyre direction can sometimes change how the tyre handles. Usually you'll see one side of the tread blocks in a directional tyre are ramped while the other side is more vertical. If you roll onto the ramped sections the rolling resistance is usually less than rolling onto the vertical sections. On my trials bike I run both tyres so the vertical part of the tread blocks on the ground points to the back of the bike to allow the tyre to hook the corners of things better - not sure if it makes a huge difference, but every bit helps...

How about the seasoned performers on here explain the following for the less seasoned:

Why 2.35cm?  Presumably you mean 2.35 inches.  Well, I've never used them.  The only reasons I can think of is if you are a downhill mentalist or just like going slowly.

Why 2.1cm? 2.1 inch, a fairly normal tyre size that works fine.

Why high tyre pressures (over 35psi)?  To go fast

Why lower tyre pressures(under 25 psi)? To get pinch flats.

Why tubeless? Less weight, lots of disposable income.

Why reverse the tyre direction? Useful to extend the life of worn tyres.

Preferred tyre thickness for winter/muddy conditions? 2.1 inch

Preferred tyre thickness for dry/summer conditions? 1.95 inch

Prefered method of puncture protection? Steering around glass etc.

Why 2.35cm?  Presumably you mean 2.35 inches.  Well, I've never used them.  The only reasons I can think of is if you are a downhill mentalist or just like going slowly.

As Karlos stated you have a greater contact patch and therefore better grip and better puncture protection. Ideal if you ride a lot on rocky trails.

Why 2.1cm? 2.1 inch, a fairly normal tyre size that works fine.

Correct 2.1 is the general racers tire width (pros prefer 1.8 or even some like 1.7). Good rolling tire width but less stable in gnarly sections and a higher risk of puncturing.

Why high tyre pressures (over 35psi)?  To go fast

Correct. But at a loss in traction on technical sections/corners. I actually ride with 40psi on most of my tires but that is because I like to go fast.

Why lower tyre pressures(under 25 psi)? To get pinch flats.

I find that riding with tires under 25psi the tire tends to deform too much sending you off line.

Why tubeless? Less weight, lots of disposable income.

Tubeless tires give you several advantages. Pinch flats will generally become a thing of the past (but not impossible to do). I have been running tubeless for several years now and I generally get one puncture per year as apposed to one per week when I was using inner tubes. If you do get a thorn puncture for example the sealant you put in the tire will fix it before you even notice. Plus you don't get sudden flats so often as you do with inner tubes as the rubber doesn't stretch to make the hole bigger as with inner tubes. The other major advantage is the tire seems to make better contact over the terrain.

Why reverse the tyre direction? Useful to extend the life of worn tyres.

A lot of tires are designed to run in one particular direction.

Preferred tyre thickness for winter/muddy conditions? 2.1 inch

There are two schools of thought. One a fat tire sits on top of the mud better a thin tire slices through the mud better.

Preferred tyre thickness for dry/summer conditions? 1.95 inch

Rubber type and amount of knobbles have a greater sway on how the tire performs than its thickness.

Preferred method of puncture protection? Steering around glass etc.

Tubeless with a sealant in the tire.

Why reverse the tyre direction? 

People do this deliberately ?  I thought it was only done by hungover fools rushing around to fix their bike up for a ride and realising it when it was back on the frame.

Nice photo LG btw: Have been there actually - nothing to write home about mind, but it's only a hamlet. 

A particular favourite! Pass this sign quite often, when going to the Lakes, but never been.

"Well cheers for that.......now anyone who comes on asking silly questions about tyres can be shown this. 

Tbh, for the beginner it is a bit of a minefield so it's good to get all the info on one page."

I know blokes that have been riding for years & still no feck all about tyres & why worry about all the silly tyre questions when most mtb forum content is plain stupid anyway...