When oracle 10 g is installed in the system ,by defult it creates around 27 user in hte database.

Except SYS and SYSTEM accounts rest all will be locked by default. which can be un-locked at installation time or later on by logging in through SYS/SYSTEM account.

Host name is the name of the database.

By default the host name is ORCL. which can also be changed at hte time of installation.

so when u login u need to pass username/password@hostname

if there is only one databse in the system then u need to pass only the username/password .Host name is optional in this scenario.

To unlock any account u need to login through SYS/SYSTEM account and unlock the specified user account.

Hope this solves your problem.