I regard units/subjects the same. The uni calls them units, I call them subjects.
Credit points are how much a subject is worth to your degree. For my degree I need 240 cp's, and each subject/unit at my uni is worth 10cp. This differs from uni to uni. For example USYD has subjects/units worth 6cp, 12cp whatever.
Adding to what Frigid said about contact hours and cp's. My uni reccomends to students that they study one hour for every cp per week. For example most of my subjects are 3 hours per week AT uni, but the uni reccomends 7 more hours OUT of uni to study that unit. Again this differs between uni's becuase all UWS subjects/units are worth 10cp.
Your degree will have CORE subjects. These are subjects/units that are compulsory for you do to in order to obtain your degree.
You may or may not also have electives. At my uni , sometimes you may have to choose from a set list of electives, or you may choose any that the uni offers.