[pdftex] Embedded Subset, Adobe Serif MM...??

[Bernhard Enders]

When I run PDFLaTeX and List All Document Fonts (with Acrobat Reader 5.0) I
notice the following cases:

1) Original Font is the same as Actual Font;
2) Actual Font has a Embedded Subset;
3) Actual Font is Adobe Serif MM;   <-- most cases

All fonts in the last case look very similar.
What's is happening here? What's the diference between the three cases
above?

Thanks in Advance,

Bernhard Enders.