Resources are assets the organization has. They are used to create value in the form of products and services. Capabilities are abilities an organization has to produce value out of those assets.
Capabilities are strongly related to experience, knowledge and information, and can be found in people, systems, processes and technologies within the organization.
As an example, think of a server that runs a corporate website. The hardware and the software are the resources; the amount of users it can serve a day, is the capability that those resources have (and by the way, maximization of that capability, is dependant of the ability of the IT specialists of the organization).
Capabilities are developed over time. Two competitors with similar resources, often differentiates fom what they can do with those resources.
This is why it is extremely important to stablish precise ways to incorporate information, experience and knowledge into the organization.
A business unit is a set of assets that are meant to create value for customers, in the form of goods and services. Customers pay for this value, and the business unit cover the costs and take a profit (ideally).
Just like a Business Unit, but the kind that specializes in the creation of value in the form of services.