Small, plastic gun-type thing that, when pointed and fired at the target, discharges 250,000,000 volts in to said 'target' (via two tiny barbed filaments) - rendering it a twitching, writhing, vomiting heap. The effects are very similar to a hangover after ten pints of Wifebeater. The Taser is supposedly harmless, i.e. no lasting effects... other than the "FUCK ME!" factor after having 80% of the National Grid's capacity injected up your arse.
Once frowned upon by the police, they've now realised that frying someone's nigga ass is a far better option than simply battering them over the swede with a metal baton or - in extreme cases - filling their guts with 9mm and thus saving all the embarrassment, complicated excuses on Newsnight and possible legal ramifications of the fiscal kind.
Once the preserve of Authorised Firearms Officers, there are now plans to dish them out to all and sundry, and it won't be long before some spotty PCSO is dishing out the voltage to some pikey filth on a housing estate near you. [Though it's more likely to be you or your missus for having underpressure tyres or suchlike.]
Legal in just about every country on planet Earth, and obtainable for around £40, Tasers - and associated stun weapons - are illegal for possession/carriage/use in the UK. Funnily enough, it doesn't stop the scrotes getting their filthies on them. No fucking surprises there then?