Machine Learning means just what it seems: It is the process by which a machine or system learns -- or optimizes -- based on the data it gathers. This form of artificial intelligence adjusts algorithms over time as more and more data is fed into them, making the systems smarter.
The impact of machine learning can be seen across the web. Machine learning helps display ad systems target better, allows email systems to block spam from inboxes, and enables search engines to make search results more relevant over time. Social sites can be improved through machine learning, too. For instance, Facebook's system learns through data on likes or tags, arguably improving the relevance of what appears in the newsfeed as time goes on.