Oracle DB export, possible charset conversion if NLS_LANG not set
The NLS_LANG parameter is used by the Oracle network layer to do character translation between the client and the database. This way the client can display all the characters in the database in a 'correct' way.

Your NLS_LANG should match the character-set of your database when you do an export. If you export a UTF-8 database in US7ASCII then you risk to loose information when you import this data back into a database. Why? The UTF-8 character-set can hold much more different characters then US7ASCII. The 'unknown' characters are replaced by a single character. In the past I have seen this happening that we lost all characters with accents. They were replaced by a '?' if I remember well.

If you import then you must be sure that the NLS_LANG is set to the value of your import file. Oracle will then do the mapping to the character-set of the database in which you import.