The term cab derives from the cabriolet, a two-wheeled, one-horse carriage often let out for hire. The development of modern taxicabs closely parallels that of automobiles. The first motorized taxicabs were electric-powered vehicles that began appearing on the streets of European and American cities in the late 1890s.
Though there has never been law requiring London's taxis to be black, they were, since the end of the Second World War, sold in a standard colour of black. This, in the 1970s gave rise within the minicab trade to the nickname 'black cab' and it has become common currency.
Taxi cabs in Edinburgh are mostly black, the iconic 'hackney carriage' like the ones you can find in London and elsewhere in the UK. They're casually referred to as 'black cabs' and can often seat a company of 5.
Hailing a cabIf you've ever hailed a taxi before, then doing so here in London is effectively the same thing – just stand on the curb, raise and hold out your arm towards a black cab that's approaching and will pass you and wait for it to stop (you don't have to whistle!).
The plural form of bus is buses. To be fair, a few dictionaries do list busses as an alternative plural form of bus. But it appears so rarely that most people would view it as a spelling error.