A Test case is a specific description of a single test to perform. A test plan usually contains many test cases that are carried out one after each other (or in a random order) by a tester.
A test case is always contained within a test unit.
Test cases are created "globally", i.e. all test units created are independent of any specific test plan. You assign test units and test cases when selecting them for a specific test plan.
Besides a description of the actions to perform, test cases can also contain an attachment, e.g. example documents to show the expected output of an application being tested.