Data. This is 'symbols that are used to represent something', an example is the set of numbers (1,4,3,7,2,7). They do not have any context to them and so are completely useless on their own. You may know this as raw data.
Information is the next stage on from data, it is 'data interpreted in some meaningful context'. This means that your numbers must actually mean something. Number of children: 1, Age:24 would be an example of a piece of information (Data with a context).
A System is a set of parts that exist to achieve a goal. An example of this is the student data system which has the purpose of being down and occasionally giving out useful information to students.
An information system is something further than a system, it is a system of communication between people. 'Information Systems are systems involved in the gathering, processing, distribution and use of information.' An example of such an information system could be the workings of a student-run society. It has the goal of providing a service to students and must manage its resources (Money, people, time) as it sees fit.
Finally, a Human Activity System is a collection of activities that a group of people will do that has specific goals.