SAP is simply an off the shelf ERP(Enterprise Resource Planning) system which you can buy from the German company SAP.SAP was started by a group of five former IBM employees in 1972. SAP stands for Systeme, Anwendungen, Produkte originally in German which means Systems, Applications and Products in English. SAP, which is holding nearly 30% market share has become the leader of the ERP market by defeating its strong competitors Oracle and MS Dynamics over last decades. SAP is a proprietary software system therefore you can't have SAP in your company or learn SAP, for free.
Main feature of SAP ERP is integrating many business functions of an organization through a common database. SAP has adopted best run business practices of the industry over its long history thus it has become a very strong and stable system. SAP has a wide range of industry specific solutions available. The list goes beyond 20. SAP Oil and SAP AFS (Apparel and Footwear Solutions) are two famous solutions. However if SAP does not have an industry specific solution for your industry (eg- Tyre) you can have your own solution by customizing the standard SAP system.
When you are going to implement SAP, first you should buy the (expensive) license from SAP to have SAP systems in your company then customizing can be done using your own SAP team or using another SAP consulting firm. However industry specific solutions will also have to be customized because it may not satisfy your requirements exactly as it is a tailor made system. ABAP,the language created by the company SAP can be used for enhancing standard SAP applications.