Angular 2 difference between core and feature modules
core
The core module contains providers for global services and can be guarded against being loaded from lazy loaded modules (as shown in your link) because this can easily cause bugs where lazy loaded modules get their own instance for global services (which is against the intention).
featureAs the name says - one module for one feature
Otherwise, a feature module is distinguished primarily by its intent.
A feature module delivers a cohesive set of functionality focused on an application business domain, a user workflow, a facility (forms, http, routing), or a collection of related utilities.
shared
This is mostly for convenience where several modules are exported so they can be made available at once in components that want to use them all (common pipes, components, and directives you probably want to use together in many other modules).