Final answer:
The key difference is that a base unit is independently defined and fundamental, while a derived unit is composed of combinations of base units.
Step-by-step explanation:
The difference between a base unit and a derived unit lies in their definitions and dependency. A base unit is a fundamental unit that is defined independently and does not rely on other units for its definition. On the contrary, a derived unit is a composite unit that is created by combining base units in accordance with algebraic relationships between the associated physical quantities. For instance, in the International System of Units (SI), there are seven base units which include units for length (meter), mass (kilogram), time (second), etc. Derived units are formed for quantities like area (square meters), volume (cubic meters), and speed (meters per second), which are combinations of these base units.