Licensing Laws as of January 2013

A guide to the classifications and restrictions and machines you need to 'learn' on to meet the required standard. Note: no scooter with the exception of the Honda 750 and Gilera GP800's meet the minimum power requirement to do direct access with, and unless you can find a training school with one as a loaner (not likely) you will have to go the geared bike for the test even if you never intend to ride anything other than autos.

If you're thinking of riding anything with an engine, you need to do the CBT first. Even long licence holding car drivers will need to do this one day basic safety course which will 'teach Grandma to suck eggs', but you may get a few insights you hadn't considered from a car driving perspective.
The CBT is valid for 2 years only then has to be re-taken unless you pass one of the full riding tests. Once done you can look into the...
Theory Test
If you're booking all your training through one company, they may be able to book this for you. If not, check the Driving Standards Agency website for details of your nearest test centre or call the DSA on 0870 010 1372 between 8am and 6pm weekdays to book.You can obtain training software CD's and books to help. The test is not the same as for a car, so even a fully qualified car driver has to re-sit the theory test. You only need pass this once to progress through any of the age related practical tests.
Practical Test
As with the theory exam, you can either book the practical test with the DSA directly, or through your instructor. The test is split into two parts, a manoeuvring test done in a controlled
environment which you have to pass first, then a road ride, during which you will be followed by the examiner in either a car or on a motorcycle. He will be in constant radio contact with you, and will give clear directions on where to ride. Throughout the test you will be marked on motorcycle control and your reaction to day-to-day road conditions. There is also a short question and answer session on elementary maintenance and safety. You do have to do both parts on a bike of similar power and size, so you cannot do part one on a 125 and part two on a 500.
The test is age related from January 2013 and no longer has the probationary 2 year time limit before you gain an automatic upgrade to ride larger bikes. As such you may have to take as many as three test if you initially start riding at the age of 16 or even 17. 
Age 16 or over - you can take a test to ride a MOPED of up to 49cc which gets you the AM category on your licence. No L-plates and can take a pillion but no motorways.
Age 17 up to 19 allows you to get the A1 category which is for up to 125cc bikes which adds pillion use and also motorways. No probation period before being able to upgrade to a larger bike. To do that you need to take the larger CC test.
Age 19 up to 23 you can do the A2 category on bikes over 395cc and between 33 and 46 HP.
Age 24 and over is the new 'Direct Access' which is taken on bikes of over 53HP or 600cc which give immediate access to any bike you wish.
And finally... For the first two years after passing the test new rider's (or car drivers) licences are restricted to just six penalty points instead of the usual 12. In other words, get caught speeding only twice, and you'll have to retake the exam to get your licence back.