Schema Patch :
Schema Patch is used to create/delete or alter database table.
Schema Patch is a class and present on the following path - vendorname/modulename/Setup/Patch/Schema/Patchname.php
Data Patch :
Data Patch is used to insert data in database table.
Data Patch is a class and present on the following path - vendorname/modulename/Setup/Patch/Data/Patchname.php