Goals are often used to represent stakeholder's objectives. The intentionality inherited by a goal drives stakeholders to pursuit the fulfillment of their goals either by themselves or by delegating this fulfillment to third parties. In Service-Oriented Computing, service client's requirements are commonly expressed in terms of inputs, outputs, pre-conditions and effects, also known as IOPE. End-users, i.e., human service clients, may have difficulties to express such requirements as they would have to deal with technical issues such as the request's language, and the type, format and coding of the IOPE. This paper presents the core concepts of the Goal-Based Service Ontology (GSO) that relates goals and services. By grounding GSO in a well-founded ontology we aim at clarifying the semantics for a set of relevant domain concepts that can support specialists in defining application ontologies based on goals and services.