RMAN : diff between duplicate and backup

I will be duplicating my Oracle8i dbs. I like to use rman. I read thru the document and found that I can use either backup or duplicate to fulfill my job. Which 1 in fact is better? What are their differences?

Thanks.
Wilson.

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Tang Wai Shing, Wilson		Email: cswstang_at_cs.ust.hk
Dept. of Computer Science, 	Tel# : (852)-2358-7004  Fax: (852)-2358-1477
HKUST, Clear Water Bay Road,    Fingerprint: 	54 8D 55 F5 B5 1E AC 7E  
Hong Kong.					80 40 23 18 8B 8C ED C4