The word jeans comes from the French phrase bleu de Gênes - literally, the "blue of Genoa", from the port where Italian sailors wore them in the 19th century. Meanwhile, denim takes its name from de Nîmes, after the Provençal town where the rough cotton twill was originally produced. From miners in the California Gold Rush of the 1850s, through to teenagers a century later in the 1950s (think Mr James Dean in Rebel Without a Cause), and Times Square billboards advertising designer pairs by the likes of Calvin Klein and Levi's, denim has maintained its status as a staple of the male wardrobe thanks to its comfort, durability and iconic associations.
Unwashed jeans (often described as raw, rigid and dry) are not pre-washed, meaning they will change in appearance over time, acquiring a broken-in look. Sanforization is a process which prevents jeans from shrinking when they are washed. Un-Sanforized and unwashed means jeans will shrink when wet, so it's advisable to buy them two sizes up if you plan to launder them. These jeans are marked "shrink to fit" and can be intentionally shrunk by submerging them in water while wearing them, to achieve a personalised fit.
Washed jeans are less complicated, but offer a less "authentic" experience (according to denim purists, at least). If laundered correctly they will not shrink significantly, and already have a worn-in look thanks to various treatments manufacturers use to wash them.
Selvedge denim is woven on narrow looms, a time-consuming process that ensures the fabric has a woven "self-sealed" edge which is durable and will not unravel. This in-built seam runs down the centre of the denim, meaning the jeans only need to be stitched down one side of each leg in production (the selvedge edge runs down the other side). This makes them stronger and can also improve the fit. Another element to consider is production location (yes, really): denim nerds will tell you that it genuinely matters whether the loom is placed on a concrete or wooden floor since the latter makes the looms move and reverberate when running, adding "character" to the denim as it is woven.
Other terms you'll encounter when shopping for jeans include those relating to the cuff: straight leg, boot cut (slightly wider at the cuff, originally to allow ventilation and so sailors could pull them off more easily if they fell overboard), flare (they have a wide aperture - aka bell bottoms). Tapered legs decrease in width towards the bottom. If this taper is fairly noticeable, and the jeans are cut narrowly at the top, then they are usually termed slim fit.