Connected lookup will receive input from the pipeline and sends output to the pipeline and can return any number of values & Unconnected lookup can return only one column.

In ETL tools like Informatica, connected lookups receive input directly from the pipeline and can return multiple values/columns back to the pipeline. Unconnected lookups are called from expressions using the :LKP reference and can only return a single value, making them suitable for simple lookups within transformations.