The terms have been used interchangeably quite a bit lately because of the popularity of disk backup that is really gaining. The thing is, disk backup is technically another form of media that your backup application or solution can write to. Data replication technically should be an exact replica (as the name implies) of your data.
The main difference here is data replication can be acquired as a volume and immediately (or almost immediately) accessed by the application, so it is written in a format that is understood by your database or your email server -- whatever your application is. However, if you backup to disk, the data will be written to a disk device in a format that is understood by the backup application. Therefore, you need to restore the data that was backed up to disk to be able to access it. That would be the main distinction here, and should not be confused, because one is almost immediately accessible (e.g., a mirror or a snapshot), where the backup implies a restore operation before the data can be accessed.
That's an important distinction because now, if we're talking about disk backup, we're talking about faster backup than tape, but not faster restores than data replication. Data replication almost allows you to do almost instant recovery of your data -- a very specific distinction between the two.
Go to the beginning of the Disaster Recovery FAQ Guide
Storage replication information