We mean several things as "core" so initially this may be confusing:

• Core modules: these are the modules that reside in Orchard's Core project (Orchard.Core). They are always installed and always enabled, so your modules can safely depend on them. Examples include Content (basic content management features like admin content lists) and Navigation.
• The Orchard Framework: Orchard is a powerful web development framework. This framework is contained in the Orchard.Framework class library. Being in the center of Orchard it's also often called "core".
• Non-core modules that are developed by The Orchard Team and maintained in the Orchard repository are also often dubbed "core modules" but technically they aren't. These modules are sometimes also called the "built-in modules". Furthermore there are such modules that have their category (in their module manifest) declared as "Core": such modules behave exactly like true Core modules (like they are always enabled) but they are not part of the Orchard.Core project.