Denim has accumulated more terminology than almost any other fabric, and most of it describes production methods rather than making claims about quality.
Selvedge
The self-finished edge of fabric produced on a shuttle loom, where the weft thread runs continuously back and forth, creating a closed edge that does not fray.
Modern projectile looms cut the weft each pass, producing a raw edge that must be overlocked.
Shuttle looms are slower and produce narrower fabric, which is why selvedge costs more.
Whether it is better denim is a separate question. The edge finish itself has no effect on wear. What correlates is that shuttle-loom denim is generally made with more attention throughout.
The visible selvedge line in the outseam when cuffed is essentially decorative, and it is what most of the interest is actually about.
Raw denim
Denim that has not been washed or treated after dyeing.
Which means it is stiff initially, will shrink on first wash, and will fade according to how it is worn rather than how it was processed.
The fading is the point. Indigo dye sits on the surface of the cotton fibre rather than penetrating it, so abrasion removes dye and exposes white core.
Which is why creases, pocket outlines and knee patterns develop into visible marks unique to the wearer.
Pre-washed denim has had this done chemically or mechanically at the factory, uniformly.
Sanforised and unsanforised
Sanforisation is a pre-shrinking treatment applied to the fabric.
Sanforised denim shrinks minimally, so the labelled size is roughly what you get.
Unsanforised, sometimes called shrink-to-fit, will shrink substantially on first wash, generally around one size.
Which means buying unsanforised requires either sizing up or soaking before wearing, and getting this wrong produces trousers that do not fit.
The label states which, and this is the single most practical piece of denim terminology.
Weight
Measured in ounces per square yard.
Around twelve ounces is midweight and comfortable year round.
Sixteen and above is heavyweight, takes considerably longer to break in, and holds crease patterns more sharply.
Twenty-plus exists and is genuinely uncomfortable for a while, which some people consider part of the appeal.
Lighter weights, under eleven, are summer weight and wear out faster.
The dye
Rope dyeing passes yarn through indigo baths repeatedly, building layers on the surface.
Natural indigo and synthetic indigo are chemically the same compound, though natural extract contains other substances that affect the shade.
Sulphur dye is sometimes used underneath to produce different fade characteristics, and combinations produce the range of colours available.
The number of dips affects depth of colour and how the fade develops.
Fades and the vocabulary around them
The wear patterns have names, which is where the subculture becomes obvious.
Whiskers at the hip creases, honeycombs behind the knee, stacks where fabric bunches at the ankle, and the outline of whatever lives in the pockets.
These develop over months of consistent wear, and the common advice to avoid washing for long periods is where I would push back.
Not washing does produce sharper contrast, and it also degrades the fabric, since dirt particles are abrasive and perspiration is acidic.
Washing gently every month or two produces slightly softer fades and considerably longer-lasting trousers, which seems the better trade.
Stretch
Elastane added for comfort and fit, now very common.
It fundamentally changes the fabric — it will not fade the same way, it will lose shape over time as the elastane degrades, and it cannot be repaired as readily.
Which is a real trade for comfort, and it is worth making knowingly rather than by default.
Where it is made
Japanese mills acquired old shuttle looms when western manufacturers moved to faster machinery, which is why so much selvedge denim originates there.
Mills in the United States, Italy, Turkey and elsewhere produce selvedge as well, with different characteristics.
Country of origin on the fabric and country of manufacture of the garment are separate facts, and marketing frequently blurs them.
What actually matters
Fit, weight suitable for your climate, and whether you will wear them enough for any of the rest to be relevant.
Everything else in this vocabulary is interesting and is genuinely secondary to those three.
Cut and the terminology around it
Where most of the actual purchasing decision sits.
Rise determines where the waistband sits and it changes the proportion of the whole outfit more than any other measurement.
Leg opening at the hem, thigh measurement and knee measurement together define the silhouette, and brands publish these inconsistently.
The terms — slim, tapered, straight, relaxed — are proprietary and mean different things at every brand, exactly as clothing sizes do.
Measuring a pair that fits and comparing to published measurements is the only reliable approach.
Repairs
Denim is among the most repairable garments in common use, which is part of why the culture around it developed.
Darning, where thread is woven across a thinning area before it holes, preserves the fabric and is nearly invisible when done on the inside.
Patching from behind with a similar-weight fabric and stitching across it handles actual holes and produces a visible mark that most wearers come to prefer.
Specialist repair services exist that reweave denim to match the original, which is expensive and remarkable, and ordinary alteration shops handle most of it competently for very little.