I'm not sure of the difference. While optical kerning may perhaps be a handy tool for large bodies of text, for display type (and wherever possible) one should always manually kern.
I guess metric kerning would be based on the kerning info from the font file, where as optical is an approximation/adjustment based on the visual character. Still never going to be as good as adjusting by eye.

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Paul Ducco
Solid Creative
Communication Design, Melbourne

Paul, you got that right.

Personally, I wouldn't use optical kerning for anything other than display sizes (usally do it by eye anywway). An exception would be made with badly spaced fonts which can look better in text settings with the Adobe's optical kerning.

Also may be of interest;
Opinions on Optical Kerning
IKern vs Adobe InDesign's optical kerning

Folks,

All kerning should be optical.

Metric kerning was a “cheap shot” invented to improve the work of "secretaries with minimal typewriter skills". What actually happened "is you became one of them".

Did I say that? Bite my tongue!

Gerald Giampa

Wait ... you want US to bite YOUR tounge? ... OK!!

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Paul Ducco
Solid Creative
Communication Design, Melbourne

-edit (disregard this whole post! I just saw Geoff's links. Thanks. There's my answers)

Thanks guys, but i dont think anyones actually answered the question...

I know there is no substitute for kerning by eye.
Metric kerning uses the kerning info stored in the font file, but what info does the 'optical' setting in Adobe kerning use to make its adjustments.

I guess i should RTFM...

ok, i did.

'optical' kerning in CS is used To automatically adjust the spacing between selected characters based on their shapes.

Hm, see, isn't that what typographer's do when they write metrics for their typefaces?

Sounds like 'optical' kerning is an approximation for faces with crappy kerning info.