What are "clip-ons"?
Clipons are handlebars that clamp individually to each fork tube, rather than a single handlebar held by clamps. Racers and sport-bikers prefer them because they are stronger than tube handlebars, are easier to adjust for position, and place you lower and closer to the front wheel, putting more weight over the front for better road feel and lower wind resistance. Also, since you can just replace one side, clip-ons are cheaper if you crash a lot. (ahem)
The EX250 has (soft, pot-metal that bend easily) bars that bolt into risers bolted to the top triple-clamp. For a better idea of what we're talking about, look at the parts microfiche for the EX250 on buykawasaki.com, then look at a bike with real clip-ons, like the ZX6-R.
Many new riders do not like the low position clip-ons provide, since you need to build up muscles in your arms, back and wrist to get used to it. The 250's position is much more comfortable for new riders and shorties.
How does a "carburetor" work?
The Ninja uses a Constant Velocity Carburetor, which means the velocity (speed) the air flows through the carb remains constant. However, the volume of air moving through changes as the slide raises. It's a mutual relationship; the more air that's drawn through, the more vacuum there is to pull up on the diaphram, thus raising the slide and allowing a greater volume of air through.
The actual throttle cable is connected to a metal butterfly valve which resides in the carb throat just inside the slide. As the butterfly opens, more air is allowed to enter. The Jet Needle, which is attached to the bottom of the slide, raises out of the Needle Jet as the slide raises. Thus, more air -> more gas. The needle has a precise taper to it, being thick at the top and thin at the bottom. This taper is crucial to the bike having good power through the RPM range. This is where all the money is spent buying a Factory Pro jet kit. Their needle has an excellent taper for the 250's needs. As the needle is raised to its highest point it no longer has a part in controlling mixture, and the job goes to the Main Jet which resides at the bottom of the tube from which the Needle Jet was raised. This is the maximum amount of fuel which can be delivered to your engine. Thus, the main jet affects your engine's performance at the higher RPM range, and is critical in tuning your engine to deliver maximum power.
The point of this is to deliver the optimum mixture of air and gas at every RPM. The process is difficult - which leads to your carburetor having the three different fuel circuits which are each used for a different part of the RPM range (pilot, needle, main). Thus the invention of fuel injection, in which this is all figured out by a computer.
See also: How a carburetor works: Carbs explained.
What is "jetting"?
Carburetor jetting is the fine art of adjusting the jets inside your carburetors to provide optimal fuel flow into your engine. Jetting is usually only done if an aftermarket exhaust or intake system is added in place of the stock ones.
What is "carb sync"?
Carb sync is matching the butterfly valves in the two carburetors so they both open the same amount. Your bike will not run at its optimum level if one cylinder is running harder than the other.
The easiest way to understand how the adjustment works is to take your carbs off and look at the linkage (ok, so that's not so easy). There's a screw adjustment in between the two carbs that adjusts one butterfly in relation to the other. Carb sync is adjusted by measuring vacuum through each carburetor and setting the butterflies so vacuum matches. There are fancy gadgets to do this, but there's also a quick and easy way to do it at home.
How does a "choke" work?
The choke and throttle are different. The choke reduces air flow and increases fuel delivery, richening the mixture; the throttle adds fuel and air. If you have a flooded engine the worst thing to do is pull the choke. The full throttle trick works by increasing airflow to lean the mixture enough to get combustion.
If you take the choke off too soon on a cold engine and try to substitute, you'll notice that it is virtually useless; the mixture is too lean.
What is a "fairing"?
The fairing is the plastic (usually) covering on the motorcycle. There are a lot of different kinds, but the Ninja 250 is fully faired. It has a windshield, a covered engine and plastic that goes all the way down to the bottom of the bike.
A bikini fairing is a very small fairing, usually mounted directly above the headlight on an otherwise fairingless (or "naked") bike, like an SV650. It might provide minimal wind protection, or it might be just for looks.
A cafe fairing is a small handlebar-mounted fairing, usually with a rounded, bullet shape to it. The classic cafe racers of the 1960s sometimes used these small fairings. The BMW R90S is a fairly quintissential bike-with-cafe-fairing. Cafe fairings provide a small amount of wind protection, but not much.
A half fairing is one which provides some wind protection to the rider, but only goes about halfway down the height of the motorcycle. Most half fairings don't cover the engine, instead showing it off. Some half fairings are surprisingly good at wind protection for the upper half of the rider's body. A Kawasaki Z750s is an example of a bike with a half-fairing.
A full fairing is one which entirely encloses the engine, usually providing a bubble of still air for the rider, at least at freeway and higher speeds. Sport bikes and touring bikes (like the Honda Goldwing) are both said to have full fairings, but obviously they serve different purposes. A sport bike's full fairing provides a very small bubble of wind protection, suitable for ducking down and providing the smallest possible profile to the wind. A touring bike's full fairing is designed to protect the rider sitting straight up, as well as provide a much larger bubble of still air, with a concentration on the arms, hands and legs, as well as just the upper body.
What is "unsprung weight?
Unsprung weight is anything on a bike not being supported by the suspension; e.g. wheels, tires, brake rotors, chain, sprocket, swing arm. Unsprung weight is undesirable because the more weight an unsprung component has, the more inertia it has (p=mv).
The more unsprung inertia there is, the harder it is for the suspension to absorb changes in inertia by exerting a compensating force, as it has to do more work. (f=ma, p=mv, a=[change in v], conservation of momentum/energy)
So, less unsprung weight means the suspension has a faster reaction time to bumps in the road, causing less jostling for the rider. Forces not absorbed by the suspension are passed on to the components on top of the suspension, including the rider. So, the higher the ratio of sprung to unsprung weight, the smoother a ride you get.
The smoother the "intrinsic ride", the stiffer you can make the suspension without rattling the rider's teeth out.
The stiffer the suspension, the better the handling and feedback.