The difference between cutomizing and configuration is:
CONFIGURATION: we will configure the system to meet the needs of your business by using the existing data.
CUSTOMIZING: we will customise or adapt the system to your business requirements, which is the process of mapping SAP to your business process.
Also, When considering enterprise software of any type, it is important to understand the difference between configuration and customization. The crux of the difference is complexity.
Configuration uses the inherent flexibility of the enterprise software to add fields, change field names,modify drop-down lists, or add buttons. Configurations are made using powerful built-in tool sets.
Customization involves code changes to create functionality that is not available through configuration. Customization can be costly and can complicate future upgrades to the software because the code changes may not easily migrate to the new version.Wherever possible, governments should avoid customization by using configuration to meet their goals.Governments also should understand their vendor's particular terminology with regard to this issue since words like "modifications" or "extensions" often mean different things to different vendors.
configuration : is where u need to do the basic settings for a company and for a plant to run in SD terms it is per sales office wise. where in u follow only standards of SAP.like vk13 vk11 MM01 Va01
customization : is where u can provide the customer a 'Z' formed tcode as report and creating 'Z' formed forms as o/p from standard tcode assigning it to an output type created by the consultants.
u can change whatever the customer requirement and u can meet the deadlines easily.
Hope this would have clear ur doubt.