what is the different between Connected and Unconnected Lookup?

Connected Lookup	Unconnected Lookup
Receives input values directly from the pipeline.	Receives input values from the result of a :LKP expression in another transformation.
We can use a dynamic or static cache.	We can use a static cache.
Cache includes all lookup columns used in the mapping.	Cache includes all lookup/output ports in the lookup condition and the lookup/return port.
If there is no match for the lookup condition, the Power Center Server returns the default value for all output ports.	If there is no match for the lookup condition, the Power Center Server returns NULL.
If there is a match for the lookup condition, the Power Center Server returns the result of the lookup condition for all lookup/output ports.	If there is a match for the lookup condition,the Power Center Server returns the result of the lookup condition into the return port.
Pass multiple output values to another transformation.	Pass one output value to another transformation.
Supports user-defined default values	Does not support user-defined default values.

Thanks 

Ur's Hari