Artificial Intelligence: Modern Magic or Dangerous Future?

Artificial intelligence has long been a mainstay of science fiction and increasingly it feels as if AI is entering our everyday lives, with technology like Apple's Siri now prominent, and self-driving cars almost upon us.
But what do we actually mean when we talk about 'AI'? Are the sentient machines of 2001 or The Matrix a real possibility or will real-world artificial intelligence look and feel very different? What has it done for us so far? And what technologies could it yield in the fut...