what's the diff between 'aptitude upgrade' and 'aptitude dist-upgrade'?

[cwinl]

hi all,
i'm confused about several aptitude param.
thank you all.

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P4/2.4B
ELSA 528/128
512MB DDR333
240GB
MSI 845PE
BenQ FP557s
Pioneer DVD-120A
LG GCE-8320B

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http://www.cwinl.com/
http://forum.lnnu.edu.cn/