can one give general differances on online and oflline backup

The basic difference is during offline backups you cannot access your database.....whereas Online backups can run while the database is still in service,

An offline backup assumes that no user or process is connected to the database except for the one performing the backup. Hence during offline backups no other user can access the DB
Online backups require only that the tablespaces being backed up not have a user (other than the one performing the backup) connected to them.This is especially useful in a production environment where you cannot afford a downtime

big difference is also for the restore
with offline backup : the database can be made available, even if the log files (created after the backup - for rollfoward) are not available
with online backup : you need the log files, active at backup time to be able to make the database available;
or you have to use include logs with the backup command to be able to leave the rollforward state of the restored database
anyhow : also backup the log files