SAP Basis is that the core technical element of SAP System. It has to try to to with the SAP Netweaver Application Server monitoring and Administration. It includes installing SAP Systems, do technical settings like setting up Servers/Server Instances, System users Management, Memory Management, underlying management. This is basically SAP System Administration in Technical terms. This is the job of SAP System Admin.
SAP ABAP( Advanced Business Application Programming) may be a programming language like Java/J2EE ..used to produce Business Applications.
SAP Applications like ERP,CRM,SRM,SCM,PLM are developed(by SAP) and enhanced (by Partners/Customers) using ABAP.SAP Netweaver Application Server provides the development tools( like code editor, compiler, runtime environment) for developing and running applications in ABAP. This is the Job of SAP ABAP Developer/Programmer.