This is a letter straight from Panaracer.  I asked them for help on the
topic of tire info since they are one of the more established companies in
the tire market.  Please do not think that I'm in any way associated with
the company nor do I get profits from this little article.  All I know is
that the article has some great info on tires and, in this case, I'm not
going to remove the product labels.  Remember that this info applies to all
brands of tires, not just to Panaracer.

Panaracer [Panaracerx@aol.com]
This is a response to your request for tire information for your FAQ on
mountain biking.  Although this e-mail attempts to answer your specific
questions, this is not the be-all and end-all on tire information.  There
are issues that are way more complex than what we've written here.  For most
people, this level of information is fine. 

[personal info snipped by Vincent Cheng, the author of the FAQ]

Choosing a tire:
There are so many tires on the market that it is possible to choose a tire
that's designed specifically for 1) the kind of terrain you ride and 2) the
kind of rider you are.  Most tires are marketed for certain uses, and if
you're shopping around, these quick descriptions are a good reference
point.
For example, if a tire has "SC" (for "soft condition") in its name, it's a
good bet that it will work best in sand or mud. 
Generally, for looser conditions, a tire should have a more open tread-that
is, more space between the knobs.  When we created the ultimate mud tire,
the Spike, Panaracer established three main concepts in good mud tire
design: smaller, non-block knobs, sufficient gaps between knobs, and a
hard-rubber compound.  This design prevents mud from clogging the tire. 
The best way to choose a tire is personal experience-make an honest
appraisal of the kind of riding you do, then see how well certain tires
work for you.  To help out this process, you can check with experienced
riders in your neck of the woods.  Keep in mind that your tire needs change
during the year to account for different ground conditions.  Also, there are
many different considerations in a tire's performance-all of which compete
for your attention.  Following are some of these considerations and why they
matter. 

Compound difference:
The black in black tires comes from the existence of carbon black in the
compound.  Carbon black gives the tire extra hardness and durability. 
Even among black tires, there are differences in the density of the tread
rubber.  Low-density rubber compounds are another shortcut to low weight,
but they come at the cost of tire performance and durability.
In recent years mountain bikers have become familiar with so-called
soft-compound tires.  These tires are usually distinguished by their color
treads.  In most cases, these tires lack carbon black in their compounds.  As
a result, their treads are quite flexible, which enables the tire to mold to
the trail, thereby improving traction in a lot of conditions.  The downside to
this is that the tire can be squishy under load and wear out pretty quickly. 
Panaracer has developed several ways to harness the benefits of soft-compound
tires while minimizing the drawbacks.  The Hard-Core tires have knobs that
combine a hard interior with a soft, grippy outer layer, giving exceptional
grip without the usual tread instability or distortion.  The Magic tires use
a unique Binary Function Compound that makes the tread stable in the rolling
direction and compliant on corners, where you need extra gripping power.  As
a leading tire manufacturer, Panaracer will continue to develop new compounds
that expand the gripping ability of MTB tires. 

[words from the author]
There are many companies out there that are making the dual-compound tires.
These tires have a softer compound in the middle and a harder compound on 
the side.  These are nice tires as well.  Remember that no matter who 
makes the tires, the softer compound tires (in this case the Magic tires) 
do wear down quicker.

Some added:
J. Wesley Prince [wesprince@csra.net]
I was referring of course to Slick Rock in Moab, petrified sand dunes
are the only slick rock I know of and the softer compounds do work better on
them but don't hold a candle to a completely slick tire, with which you can
defy gravity on slick rock.

Thread count:
High-density nylon cord is the choice material for tire casings.  A higher
thread count (like 127 TPI, or threads per inch) indicates a denser weave,
which improves resilience.  Some companies use a thinner nylon cord, which is
not as tough as regular nylon cord.  Compared to regular cord, thinner cord
does not do as good a job of withstanding forces like hard cornering and
sharp objects.  Thinner cord is lighter, but that's not an acceptable means of
low weight. (Panaracer tires use regular high-density nylon cord.)

Tread shape:
The functions of front and rear tires are completely different.  A rear tire's
main functions are driving power and braking, while a front tire's main
functions are cornering and steering.  It follows that the shapes of the
primary knobs are also different--rear tires use paddle-shaped knobs, while
front tires counter with arrow-shaped knobs. (Panaracer established the
front/rear patterns with the Smoke/Dart combo.)
The complete tread pattern should be designed to give the maximum ground
contact at any angle--from straight-on riding to hard cornering. 
Some tires alternate the height of the knobs, so that some knobs hook up on
hard surfaces, then the others hook up in loose conditions. 
Some tires use fewer knobs as a route to low weight.  This usually
compromises tire performance. 

[words from the author]
There are tires out there that perform very well with very little knobs, 
such as many of the Conti tires.  Remember, Panaracer is telling us a 
lot about tires, but all from their own research.  While they may be 
correct, there are sometimes alternatives to their "methods."

Kevlar/steel bead:
Ounce for ounce, Kevlar is five times stronger than steel.  This high strength
makes Kevlar a good bead material.  Kevlar beads are foldable, and they save a
lot of weight.  All Panaracer mountain bike tires, and many other mountain
bike tires, are available with both folding and steel beads--the difference is
90 grams per tire.  This weight savings does not sacrifice performance in any
way-and if light weight is an important consideration to you, folding-bead
tires can improve your rides.
One other bead note: On our new DusterPro tire, Panaracer has developed a
special bead covering, an elastomer material that helps prevent pinch flats.
This strip absorbs shock, so when your tire compresses over a sharp bump, the
tube does not bottom out on the rim, thereby avoiding a pinch flat (otherwise
known as a "snakebite"). 

GreenLite tube:
Lightweight inner tubes represent an efficient way to save weight. Because
this is rotating weight, it is all the more significant in performance. As
for durability, it depends--for some riders, light tubes are often superior
in quality and durability to the standard butyl tubes they replace. 
There are several paths to lightweight inner tubes. One is a lightweight
butyl tube--but because this is just a butyl tube with thinner rubber, it is
not as resilient.  Another lightweight tube material is latex. Desirable for
its stretchiness, latex can help give a supple ride.  But it also has its
problems: latex can be fragile and porous, and in the event of a flat, it's
not easily repairable. 
Panaracer came up with something better in the GreenLite tube.  It uses an
all-new tube material: a strong, supple grade of urethane.  The GreenLite's
low 90-gram weight puts it up there with the lightest MTB tubes.  It's supple
feel helps smooth out the ride.  It does not leak air overnight like latex,
nor is it as prone to failure.  And it is patchable with glueless patches.

Some added:
J. Wesley Prince [wesprince@csra.net]
Yes latex can be fragile, as it has short life of optimal strength and
must be replaced after one season, sooner if exposed to heat.  They are quite
wrong however in saying it is not easily repairable.  It is in fact the
easiest tube available to repair since the surface requires no prep besides
removing talc.  It sticks to patch glue much better than butyl.

[words from the author]
I agree with Mr. Prince here.  Latex tubes can be patched, assuming that you 
remove all the talc powder with rubbing alcohol.  Remember, glueless patches
do not last very long.  Please don't use them as permanent patches.