"Main" is the one and only connection you can build from an input component to your o/p component via tmap or any other components.
every second input connection you make to your intermediate(tmap) or o/p component falls under lookup category.
Refer the below link for more information on how lookup works
Thank you for your answer.
My job's main and Lookup are using joblet as a data source,but the Main joblet must use a tOracleConnection or other DB connection component to read the data,therefore,I want to know the difference between lookup and main.