Business Component section in quality center allows you to create reusable business function/any functionality of the product/application (scripted via QTP/mercury product). It helps business users to test easily without the knowledge of QTP , all the user needs to do is drag and drop the component as per the business flow and run the components.
Do you have BPT..?? because i have tried to look for simple answers to the question you posed, its always not satisfying..so if you have a BPT license then on the job learning would be my suggestion...
Business Process Testing is a solution by Mercury to accelerate the automation of test cases. The idea is to have wireframes or screen captures provided in a use case to build components that can be build upon. Like for eg. A use case may state "users with credit rating 2 are not allowed to log in."
To validate this the automation script would need to work with the browser, the page, the username text box, the password and the submit button along with an error message.
The script cannot be made until the application is ready. a subject matter expert can come in here and define the steps using these objects which are not actual objects but dummy ones. Once the application is ready the automation engineer would create all these objects and using them business process like Login / Logout can be made.
Using these business process test cases can be designed.
The other way round can also be possible that the Automation engineer creates the objects and those are used by the subject matter experts to create the Business Processes.
To start with BPT you may download the trial version of QC and work on it.