nopPlus 

The Nop.Core project contains a set of core classes for nopCommerce, such as caching, events, helpers, and business objects (for example, Order and Customer entities).

Nop.Web.Framework is a class library project containing some common presentation things for Nop.Web project

The Nop.Data project contains a set of classes and functions for reading from and writing to a database or other data store. It helps separate data-access logic from your business objects.

This project contains a set of core services, business logic, validations or calculations related with the data, if needed. Some people call it Business Access Layer (BAL).