Becoming a taxi driver in London takes more than two years to prepare for on average, and the test itself is actually called "The Knowledge"

In the United Kingdom, taxis are called Hackney carriages. These are the only taxis allowed to pick up passengers off the street. This is to make sure every driver has the proper "knowledge" of the streets they're driving through. In fact, in London they have to pass a test called "The Knowledge" to be allowed to drive one of these taxis.

The Knowledge is a very hard test. It is the world's most demanding training course for taxicab drivers, and applicants will usually need at least 12 appearances to finally pass. Preparation averages 34 months.

So what's the point of this test? The taxicab driver is required to be able to decide routes immediately in response to a passenger's request or traffic conditions, rather than stopping to look at a map, relying on satellite navigation or asking a controller by radio. It's thought that black cab drivers are experts on London, with intimate knowledge of the city.

There are two types of badges attainable: a yellow one for the suburban areas and a green one for all of London. The latter is considered far more difficult.

(Source)