Apple is worth a trillion dollars (or 767 billion pounds, but that doesn't quite have the same ring to it). They're the first publicly listed company to reach that target.
What it means: Apple was founded in 1976, with a mission of making computers 'cheap, small, and simple'. For a while, it didn't go very well - Jobs was forced out in 1985, and in 1997 the company was on the brink of shutting down.
The short history of what happened next is that they brought Jobs back in, invented the iPhone, and made billions. The company's value has increased by 50,000% since they started selling shares, compared to an average increase of 2,000% among the US' top 500 companies over the same time period.
CEO Tim Cook, who took over after Steve Jobs passed away, is credited for a lot of the company's business success: he was responsible for managing the company's relationship with Chinese manufacturers, allowing them to produce on a much larger and cheaper scale.