Wikipedia defines a boy racer as -
Boy racer is now a New Zealand, British and Irish colloquial term for a person, usually a male in his late teens or early twenties, who drives his car in an intimidating or dangerous way for fun. Their cars are most often regular models customised with spoilers, wide tires, flared fenders and go-faster stripes. Cars driven by boy racers are often small, and it's not uncommon that they are "all show and no go", in that the cars are not the highest performance model in the range or would not be considered competitive against "genuine" sports cars. Limited funds, high insurance rates that come with young age and inexperience at driving are reasons why a person might prefer to buy a base level, 1-litre Peugeot 106 and equip it with a bodykit and sports exhaust rather than simply buying a harder to find and much more expensive to insure 106 GTi.
An example of a boy racer car of the old generation is the Ford Escort (both the early rear wheel drive versions such as the RS2000 and the later XR3i variants) due to their low cost and easily tuned engine, while modern boy racers generally use hot hatchbacks such as the Renault Clio and the Nova SR.
According to the stereotype the new generation doesn't know much about tuning the engine, though there is little evidence to prove that this is true of the majority of people who would be categorised as "boy racers". Not everyone who owns modified cars are willing to spend thousands of pounds/dollars on the engine.
Movies like The Fast and the Furious and MTV's Pimp My Ride have caused additional interest in boy racing. Boy racers tend to be dictated by trends from popular racer medias like Max Power and Fast Car, spending lots of money to keep up with the trends. Recently they can been seen moving away from The Fast and The Furious influenced Jap Style into the drifting bandwagon largely hyped by magazines, despite some owning unsuitable FWD cars, causing a lot of displeasure amongst genuine drifting enthusiasts.
Typical upgrades for a boy racer include large stereos (which often take up the whole boot and can be heard playing dance music or happy hardcore from down the road); extravagant paint jobs; oversized exhaust pipes; large spoilers; hood scoops on non-turbocharged cars and neon lights underneath the chassis. Lowering a car is an extremely common modification.
Such modifications are also favoured by genuine enthusiasts. Boy racers would be distinguished from these enthusiasts by the overall performance and quality of the cars they drive and the modifications the make in relation to the overall performance of their car. For example, a stereotypical boy racer might own a Vauxhall Nova 1.1 with oversized wheels and exhaust, and they might use it to drive round town centres at night showing off to the crowds of people heading for pubs and clubs, while the enthusiast might own a modified Toyota Supra which they drive only at weekends.
Boy racers might also flout most motoring laws, such as running their foglights during the day, speeding, weaving through traffic, cornering too hard etc. Challenging superior sports cars is another dangerous pastime which seems extremely common; it is almost an occupational hazard for a sports car driver in the UK to be challenged at the lights by someone in a small hatchback.
Not all young drivers with modified cars fall into this bracket, with many driving their cars in a sensible manner. Car modifiers and cruisers who take a pride in their vehicles are generally well behaved and often have respect for other road users. There is often no distinction in the mainsteam media between the boy racer and modified car enthusiast which can lead to the latter being tarred with the same brush. In essence the boy racer is to the car scene what hooligans are to football; however, it seems that many young male drivers are regarded as boy racers regardless of circumstance. The 'Boy racer' term itself is considered by some to be derogatory, as not all car modifiers are necessarily in their teens, or of the male gender. Most 'boy racers' do not associate themselves with the term, and instead think of themselves as car enthusiasts. Some even have a wolf in sheep's clothing style car which looks like a standard 1.1 Ford Fiesta but underneath beholds a 2.0, thus looking like a boy racer, with low profile tyres on a lowered ride. Thus showing real enthusiasm about the mechanics of a vehicle rather than the all show and now go that is usually tagged along with boy racer name.