The word technical is much overused in the outdoor trade where it is applied to just about everything sold in a bid to make it seem more glamorous. In using it for this section, we apologise, but it does sum up simply the notion of clothing designed to deal with extreme mountain weather. A "traditional" technical clothing system consists of a Base Layer of quick wicking "thermal" underwear, one or more Mid Layers - usually but not always fleece, and a Shell Layer - usually but not always waterproof.
The idea is that one varies the mid layer and may do away altogether with it and part of the base layer dependent on conditions, and in principal this system applies to just about all mountaineering situations, though there are various flies in the ointment. The main one of these is that "breathable" waterproofs do not breath well enough for active situations like walking into a crag with a big rucksack on, thus it is almost always better to wear a windproof rather than a waterproof if it is not actually raining. We have two sections devoted to such clothing, Windproofs and Soft Shell, the latter being windproofs made out of tougher, warmer, stretchier (and more expensive) fabrics than general windproofs. In fact sweat build up over several days due to not breathable enough waterproofs can lead to severe chilling when stationary - in such cases, common on expeditions, Buffalo clothing, which works as a combination of base layer, mid-layer and windproof shell combined, and does away with the need for waterproofs, is often a better solution than a layering system.In addition there are numerous options within the layering system, such as the soft shell/hard shell question and using artificial fibre instead of fleece. Another more serious problem with layering systems is that in really extreme conditions, taking an outer layer off to put on extra mid-layers may be impossible due to zips icing up etc. Using the Buffalo system or layering up but carrying a "Belay Jacket" as an item to put over the top of everything else are both possible options.