i know that i can grant a user or a role privileges to
select or update certain columns in a table but can grant
privileges for certain row?
for example, i have a table, lets call it projects and for each
row i save the id of the owner.the id is the same as the users of
the system.i want that the user that log in will be able to see only
the rows that he owns and won't be able to see the other projects
in the table which he doesn't own.
can it be done in the database level or only in application level?

You could create a VIEW (virtual table) based on the table, WHERE ID = USER. USER is the Oracle session userid of the person logged on. This should work, in which case the solution is at the database level (as the view) which is always preferable to dealing with things like this at the application level.