I started to learn machine learning in three years. When I first to learn machine learning, I do hardly to make it clear that what does machine need to learn. Actually, as a branch of artificial intelligence, machine learning is an algorithm to acquire series of parameters of the function that you want to get. We can regard a system that have an input and an output as a function, e.g. y=f(x). All what we need to do is find a function to map x to y.