Characteristics are Descriptive attributes used to describe the entities such as Customers, Vendor, Materials, Plants etc. These represent who, what, when, where scenario.
Example --> Customer Name, Address, Phone number, age, Color etc.
Master Data Characteristics are the characteristics that contain Text, Attributes and sometimes hierarchies. In general master data will be loaded into these characteristics directly using Direct Update Method.
Reference: http://help.sap.com/saphelp_nw2004s/helpdata/en/58/a4513ce4af0a40e10000000a114084/content.htm
Key Figures are operational attributes, which indicates numerical measures such as amount related, Weight related, quantity related, etc. These represent how much and how many scenario.
Example --> Sold quantity, Rejected Quantity, Stock Sales, etc.
Reference: http://help.sap.com/saphelp_nw2004s/helpdata/en/23/054e3ce0f9fe3fe10000000a114084/frameset.htm