There are three different methods available for Oracle backups. The first, a Logical Backup, copies each object definition and its data to a file. A logical backup is only a snapshot in time and offers limited recoverability options. However, the database may be up and operational while the backup is being performed. Logical backups typically use the import (imp) and export (exp) programs provided by Oracle.
An offline backup, or cold backup, shuts down the database and copies all the relevant files to a backup medium (tape). When the backup is finished, the database is restarted and users can again perform their work. While an offline backup is a snapshot in time, it can be recovered to any point in time by placing the database in archivelog mode. This type of backup is sufficient for development databases. In addition, if you have a production database that is not 24x7, a daily cold backup with archived redo logs should be your absolute minimum.
Lastly, an online backup, or hot backup, allows the database to be available as well as get a complete backup that is recoverable to any point in time. When an online backup job is running, users will typically see about a 10% performance degradation. In addition, network bandwidth is a crucial element in ensuring that users retain acceptable response times during the backup. Online backups are typically used in 24x7 shops. Hot backups can be implemented either with or without Oracle Recover Manager (RMAN). However, RMAN makes hot backups REAL easy.
Most shops employ more than one backup methodology. For example, you may want to use a combination of a hot backup and a nightly export. This way, you have two methods to recover and you can restore little "user" mistakes (user drops a table but you don't want to recover the entire database).
As with any production level procedure, the key is to TEST, TEST, and TEST your recovery. We schedule two "planned" recoveries and two "unplanned" recoveries each year to ensure the personnel know how to restore & recover.
In addition to Jeff, think of Stand By database and Multi-Master replication where you want to quickly switch over to secondary database when the primary crashes.
Most of the 24x7 OLTP system needs no loss of data.