For this topice you can through the reference manual..
Join vs Keep:
It will give the combination of two tables data(i.e single table) where as keep makes the link between two tables. But, the result will be same when you use join or keep. By using keep qlikview providing little bit future i.e it saves the memory. If you join the two tables it takes more memory.
The join and keep prefixes can be preceded by the prefix left.
If used before join it specifies that a left join should be used. The resulting table will only contain combinations of field values from the raw data tables with a full set of data from the first table.
If used before keep, it specifies that the second raw data table should be reduced to its common intersection with the first table, before being stored in QlikView.