Hi @Tina_Khone,
Just to extend the great link provided by @the_dummy, in brief Greengenes contains only reference sequences from Bacteria and Archaea. Though, this database has not been updated in quite a long while.
SILVA contains two different reference databases that include the Small subunit (SSU) and Large subunit (LSU). The SSU contains the 16S and 18S rRNA genes and the LSU contains the 23S and 28S rRNA genes. Both contain data for Archaea, Bacteria, and Eukarya. SILVA is also updated periodically.
FYI, our RESCRIPt tool allows you to easily download SSU and LSU data to make your own reference database. We have plans to allow retrieval from other databases too. Anyway, you can read more about some of our database comparisons in our preprint.
-Cheers!
-Mike