How can we describe them separately according their roles? Actually I am a new student; I don't have proper knowledge about this. Please guide me as a basic student.
Although there's a broadly accepted definition of what a Project Manager is, I'm afraid there isn't such canonical answer for what a "Functional Manager" is supposed to do. In most of the cases, it'll strongly depend on the environment or context the question comes from.
- A person responsible for a given function within a company (Sales manager, marketing manager)
- A person responsible for a given function within a project (Development manager, test manager)
- A person responsible for ensuring functional requirements are properly tracked (a kind of Functional analyst lead)
Another aspects to consider is that usually a function is something constant over time whereas a project is something with a (planned) end date.
PMBOK makes a clear distinction between these roles; just look to that. Project Manager: person assigned by the performing organization to lead the team that is responsible for achieving the project objectives.
Functional Manager: person with management authority over an organizational unit within a functional organization. The manager of any group that actually makes a product or performs a service. Sometimes called a line manager.
Another way to look at it is the Functional Manager always has a "home base" within the company, e.g., Engineering department. The Project Manager doesn't necessarily have a home base because there might not be a Project Management department, per se.