Typical features

Opened file backup
The ability to back up files that are frequently left open, such as Outlook files (*.pst) or SQL database files. This feature allows IT administrators to run backup jobs at any time of the day, not requiring server down-time. Most enterprise online backup products support an add-on for this. Higher-end products may support this natively.
Multi-platform
A multi-platform backup service can back up multiple platforms such as the various flavours of Windows, Macintosh, and Linux/Unix.
Network backup
A backup service supporting network backup can back up multiple computers , servers or Network Attached Storage appliances on a local area network from a single computer or device.
Multi-site
Some services offer the capability to back up both data at the primary site and data located at remote offices and branch offices, in a relatively seamless manner.
Continuous backup - Continuous Data Protection CDP
Allows the service to back up continuously or on a predefined schedule. Both methods have advantages and disadvantages. Most backup services are schedule-based and perform backups at a predetermined time. Some services provide continuous data backups which are used by large financial institutions and large online retailers. However, there is typically a tradeoff with performance and system resources.
Online access to files
Some services allow you to access backed-up files via a normal web browser. Many services do not provide this type of functionality.
Data compression
Data will typically be compressed with a lossless compression algorithm to minimize the amount of bandwidth used.
Differential data compression
A way to further minimize network traffic is to transfer only the binary data that has changed from one day to the next, similar to the open source file transfer service Rsync. More advanced online backup services use this method rather than transfer entire files.
Transfer encryption
Encryption to prevent interception of data. This does not mean that the data will necessarily be encrypted during storage.
End-to-end encryption
Encryption of data at the client. The encrypted data is then transmitted and stored on the server. This requires a client-generated key, and precludes data recovery should the key be lost. Depending on implementation, it may also preclude certain optimization techniques (such as recognition of common operating system files which don't need to be stored multiple times, and removal of duplicate user files across different users —'deduplication').
Bandwidth usage
User-selectable option to use more or less bandwidth; it may be possible to set this to change at various times of day.