Machine Learning is a type of AI that is best described by Tom Mitchell as: "A computer program is said to learn from experience (E) with respect to some class of tasks (T) and performance measure (P), if its performance at tasks in T, as measured by P, improves with experience E." 
Types of Machine Learning
Machine Learning is split into 2 categories; Supervised and Unsupervised Learning.
Supervised learning can be described as a type of machine learning where we already know what the output should look like. If we know the relationship of X and Y (as an example), this gives us a basis for knowing what the output will look like.
Categories of Supervised Learning
If we are given a selection of prices of cars relative to their model year, we can find out where a specified model year may result in a specific price.
In order to use a regression problem, we must have a continuous output. Price being the case here.
If we are given a selection of variables such as Age and Height, we may wish to group these variables to find out the chance of someone who is perhaps 16 years old, being a certain height or not.
In order to do this, we must classify or group inputs into categories in order to make an informed prediction.
An example of unsupervised learning is Bing or Google news.