What is the best way to get started in electric flight?
That depends on what you want to do and where you're starting from.
If you've never flown RC before, and you want to start with an electric plane rather than a pure glider, We recommend an electric glider like the Thunder Tiger Windstar EP. This will give you the best chance to stay ahead of the plane. In the sport/trainer category, I hear a lot of good things about the Leisure Amptique.
If you know how to fly RC, you have a lot of choices. The simplest and most available electric power systems use six or seven cells. These are called "05" systems, and are very similar to the power system of an RC car. You can find all sorts of planes in kit or plan form which will work well with these systems. Any two-meter glider kit can be easily adapted to six or seven cell electric by a moderately competent builder. Just stick a motor in the nose, battery under the wing, and go.
If you want more performance, good ground handling, or just like larger planes, there are larger power systems available, all the way up to systems which will handle a 60-sized power plane. The cost and complexity, of course, go up with size. Any reasonably well-designed power plane kit or plan can be adapted to an appropriately chosen electric power system. The first step is to leave out half the wood - all power planes are grossly overdesigned. Electric motors generate very little vibration, which helps you get away with lighter structures.

What are the elements of an electric power system?
The power system includes a battery, a motor, a control, and wiring. The battery is almost always made up of Sanyo NiCd cells in the appropriate number. Motors vary from the simple, cheap "can" type (otherwise known as "540" or "550" style), through more sophisticated styles adapted from RC car motors, up to the cobalt powerhouses.
Controls can be a simple on-off switch controlled by a servo, a directly controlled on-off switch, or a proportional electronic control.
If you are going to fly a glider or old-timer type plane with less than a 500-watt motor, think seriously about getting battery packs made of Sanyo 1800 SCR cells. They are significantly lighter than the more usual 1200 mAH (sub-C) cells and give excellent performance.

What do the various letters used to refer to NiCd cells mean?
SC is the basic cell. SC cells will take fast charging and have reasonably low internal resistance. SCR cells have lower internal resistance and a somewhat flatter discharge curve, that is, they put out nearly the same voltage from beginning to end of the discharge. SCRs are best for high current drain applications. SCE cells have somewhat more capacity for the same physical size, but also have higher internal resistance. They are best for low current drain use (less than about 10 Amps.) The higher capacity of SCE cells will not be realized at high current drains, and they will heat up more than SCR cells.
Many kits nowadays come with a power system. In most cases, these systems are adequate for the application. It won't hurt to try what's there to start with, you can always experiment later. If the kit you choose doesn't come with a motor, of course, you'll have to choose one. If you are a beginner, go with the recommendations of the kit manufacturer. If you are an experienced RCer, you probably don't need our help.
For a six- or seven-cell glider or old-timer with a cheap motor, an on-off switch is sufficient control. For anything else, you will have much greater enjoyment with a proportional throttle. Get a high-rate control, they are much more efficient at part throttle.

What support equipment do I need?
You need a charger of some sort. If you are using six or seven cells, any RC car charger will do the job. You don't need peak detecting or any of that fancy stuff to start with. For larger packs, there are good high-voltage chargers around. Remember, the biggest enemy of NiCads is heat, so try and keep those batteries cool when charging. Expect to pay about £20.

How are motor sizes specified?
Motors are traditionally specified by a system which attempts to equate them to wet engines. There are significant problems with this, but they probably aren't of concern to beginners. An "05" motor takes a six or seven cell battery and puts out 75 to 120 watts, and so on up to a "60" which takes 28 cells and puts out 1200 watts. Incidentally, there are about 750 watts in a horsepower.
The actual power output for a given voltage (number of cells) depends on the load. Unlike wet engines, electric motors put out more power with more load. If you don't like the performance you get from your plane, you can try a bigger propellor - up to a point. More power, of course, means less run time.
In the ideal world, motors would be specified by the total power they are capable of supporting and by the number of cells (or voltage) with which that power is produced.

What's a cobalt motor and why would I want one?
Rare-earth magnets, of which the most common type is samarium cobalt, are stronger for a given weight and volume than ferrite magnets. Perhaps an even more important reason for getting a cobalt motor is that they also have better brushes, bigger shafts, better bearings, are built more carefully, and so on. For the serious electric flier, they are worth the extra expense.

Where can I get this stuff?
Electric equipment is somewhat specialized, and most hobby shops aren't yet sufficiently enlightened to carry very much. You can use RC car equipment for a lot of things (after all, they developed this stuff in the first place) and your local hobby shop (Streamline Sales in St-Leonards-on-Sea) will have lots of that. If you want to get more sophisticated, get the catalogues from SMC. SMC catalog contains a lot of detailed information that I can't fit in here. SMC has good prices on mainstream stuff.