The scope of a project refers to the totality of products / deliverables (and their associated requirements, features and quality criteria) which are agreed between the customer and supplier (whether internal or external) before project initiation.
In traditional project management, the tools to describe a project's scope (in terms of it's existence as a product) are the product breakdown structure and product descriptions. The primary tool to describe a project's scope (in terms of the work necessary to complete the product and any additional activities required to deliver the project e.g. management activities, quality assurance etc) is the work breakdown structure.
One of the main reasons for projects not meeting expectations is cited (widely by the project management community) as a lack of clear requirements and quality definition at the initiation stage of the project and no change control mechanism (that is supported by all key stakeholders) to effectively manage the inevitably dynamic nature of baselined requirements on large programmes.
Scope management is the process of managing how the project scope will be defined, managed, controlled, verified and communicated to the project team and stakeholders/customers. A robust Scope Management process will use a number of core systems/documents to control the definition of scope and any changes to it over time. These documents are used to control what is in and out of scope of the project by the use of a Change Management system. Items deemed out of scope go directly through the change control process and are not automatically added to the project work items.