Authentication and Authorization are two terms which you may have come across while logging into a system. Even though they appear to be similar but in real terms they are actually very different!
Authentication is the first step which occurs when you try to log into a system. The System tries to verify your Identity and make sure that you are the right person to access the system. This is done by asking the user to enter their credentials. In case for some websites, along with entering your credentials, you are also asked to fill out a CAPTCHA sequence to make sure that you are not a machine. These systems have become more sophisticated and now Include Identifying pictures. For Example, it would give you a series of pictures and ask you to Identify the ones which has a beach. Once it makes sure you are the right person, you would be able to access the system.
Authorization is a step which comes after Authentication. It basically determines what are you supposed to do once you login. For Example if there are some confidential files in the server, you might not be authorized to access or modify those documents. It is basically a set of rights which are given to a user.