Test Case is a collection of test steps and a test step contains either a Keyword, a Control Flow Construct, Function Library file or Service Repository file. A Keyword can take either no input or can take object, data, or both as inputs. Each test step also returns an output which can be used as input for the other step.
A Test Case can be executed individually by using 'Run Now' and 'Debug Now' button in OpKey.
You can run multiple Test Cases together by adding them to a Suite and then executing the Suite.