* Jerseys are special shirts made from lycra or other synthetic material. They come in long-sleeved, short-sleeved, and sleeveless variants. Most have three pockets in the back that are very convenient for spare tubes, bananas, keys, and other small items (but not paper, because it gets sweaty, or large plastic items that block sweat evaporation).
Although it may sound unpleasant to wear a plastic shirt, the big advantage of jerseys over T shirts is the fact that they transport sweat from the skin to the outside very efficiently. It takes very hard effort (or a backpack) to get a jersey sweaty, while a T shirt would be soaked very quickly. This is especially important in wet weather because a soaked shirt feels much colder than a jersey. The sweat transport is less effective for very thick jerseys, so it is important to choose the right one and not ``overdress''.
To some degree temperature can be regulated by stuffing the jersey into the pants (which is not normally done) or adjusting the zippers. I prefer jerseys with long zippers. All jerseys have a rather high collar in the back to prevent a sunburned neck, and some can be zipped up the neck in front to protect against cold headwinds.
* Undershirts such as the ones made by Odlo are a very effective addition to jerseys. They can be worn underneath the jersey in cold weather, which works better than choosing a thicker jersey. The material is amazingly thin, which means they can be rolled up to a very small bundle and carried in a jersey pocket. They do not work so well if worn without a jersey over them, and they need more frequent washing because they soak more easily.
* Fleece jackets work well in cold weather. I have never found a combination of undershirts, long-sleeved jerseys, and a fleece jacket to be too cold even below -20 degrees C. Bicycle fleece jackets have water-repellant panels in front where one is exposed to wet headwind, and are more close-fitting than non-bicycle fleece jackets. As always, air pockets reduce the effectiveness. I am very happy with my Cannondale fleece jacket.
* Rain jackets come in cheap plastic bag versions that seem to get as wet inside as outside because of sweat, and expensive ones from Goretex that allow some degree of sweat transport from the inside to the outside. The Goretex version is worth its price because it keeps one dry if one does not ride too hard. The cheap plastic bag jackets work better when they have slits in the back, and if they are smoothly rubberized inside and not just tightly woven because otherwise they only last for a few minutes in the rain. Good rain jackets reach very low in the back so you can sit on them to protect from spray from the rear wheel.
See also an article on "Shoes"
