Difference between database dump (export) and database backup?
I'll answer this at a high level for you. The two backup methods work at different levels. An RMAN backup is a physical backup and a Data Pump backup is a logical backup.

A database dump using expdp is a 1-time export of one or more database schemas. It backs up DDL (table structures, views, synonyms, stored procedures, packages, etc), plus data.

An RMAN backup is a point-in-time backup of an entire database (for the purposes of this question). It backs up the physical blocks that make up the database (data files, control file, archive logs etc) and, in combination with the database archive logs, allows point in time recovery options.

In the event of a complete database loss an RMAN backup can be used to restore the complete database. However, a data dump taken using expdp would need a new database creating before the data could be imported using impdp.

For a hobbyist Oracle XE database (that may not be in archivelog mode), backups using expdp will probably suffice.

The Oracle Documentation covers this far better than I could ever explain. Oracle® Database Concepts - Backup and Recovery