It is a good idea to have separate users for each database, and to limit the hostname.
The default syntax:
CREATE USER 'username'@'hostname' IDENTIFIED BY 'password';
CREATE USER 'test'@'localhost' IDENTIFIED BY 'test';
The above will create user test, allow it to connect from localhost (connecting to a DB that is on the same machine, and has password test).
Now let’s grant all privileges on some database to the above user:
GRANT ALL PRIVILEGES ON database.* TO 'username'@'hostname' WITH GRANT OPTION;
GRANT ALL PRIVILEGES ON test.* TO 'test'@'localhost' WITH GRANT OPTION;
The above will grant all privileges on database test to user test when he connects from localhost. You can fine grain the permissions depending on username/hostname.
To check the permissions for some username/hostname:
SHOW GRANTS FOR 'username'@'hostname';
I recommend the official guide, if you need better understanding of the above.