All the data in any system can be categorised into three different groups which define the kind of data setup required to run, control and manage the applications in a way which is very efficient for memory handling.
Master Data is the control information required to decide how transaction data gets posted into various accounts (such as customers, vendors, GL, etc.). The master data is usually shared across modules (for example, customer master records are common both to FI and SD in SAP) obviating the need for defining it in various application areas. The master data remains in the system for fairly a long period.
Transaction Data is the day-to-day recording of business information such as purchase orders, sales returns, invoices, payments, collections, etc. This includes both system-generated data (tax, discount, etc., automatically calculated by the system during document posting) as well as user generated data.
Customizing Data refers to the customized information for a particular Client. This includes data such as payment terms, discounts, pricing, tolerance limits, etc., which you do not normally change on a day-to-day basis.