In Talend there is nothing called as 'lookup connection' .All the input components has a main row connection . For example , if you connect a input component to tmap , it will have a main row connection and if you connect another input component to the same tmap (rightclicking - row - main) , it will be named as lookup .
On the whole for the input connection named as main we are 'looking up' the data with data of another input , so as to pull some extra information (columns) along with the data of main connected input using some common data columns.
In definition we are looking up with another input for some extra data for the main data using some common column .
A Main row entering a tMap is the source that drives the data through the component. The Lookup is used to compare/reference to that Main row. If you Main input has 10 rows, then the tMap will consume each of those rows and use the Lookup against each of those rows. If the Main has 0 rows and the Lookup has 100 rows, no rows will be consumed and no rows will be returned. This is why you cannot carry out a full outer join with a single tMap.
The answers related to the query related to main and lookup can be verified in below KB article. Its a four part series and my recommendation will be to go through the full series. But if time constratinst are there, please refer the section related to tmap.
