I am copying this from Stephen Downes' ever informative OLDaily newsletter digest. It features an article entitled What is machine learning? – A beginner’s guide posted on the FutureLearn website.
This is quite a good introduction to machine learning. If you don't know what it is and would like a quick no-nonsense introduction, this is it. Machine learning is depicted "as the science of getting computers to learn automatically." It's a type of artificial intelligence, which means essentially that they are software systems that "operate in an intentional, intelligent, and adaptive manner." The third point is the most important, because it means they can change their programming based on experience and changing circumstances. The article talks about some types of machine learning systems and outlines some application in the field. It's FutureLearn, so at the end it recommends some course tracks for people interested in making this a career, and just to dangle a carrot, the web page lets you know the median base salary and number of job opening for the program in question.