Here are two identical pages, styled with the same values, one with pixels and the other with ems and percentages. You can also view the pixel style sheet and the ems and percentages style sheet.

Here is the preferences dialog box, where your visitors choose the default size for body text in Netscape. Here it's set to 16 pixels.
Default font settings in Netscape, set to 16 pixels

And next I show you the same document, one with the styles defined with pixels (on the left) and one with the styles defined with ems and percentages (on the right). This is Netscape.
Netscape, with default size set to 16 pixels

Explorer, whose default body text size is also 16 pixels, but which doesn't let the visitors set a different size, looks like this:
Explorer for Windows with default body text set to 16 pixels

The difference is that in Netscape, your visitors might choose a different default size for body text:
Setting default text size to 20 pixels in Netscape
and then the page with sizes set in pixels (left) stays the same, while the page with sizes set in ems and percentages (right) adapts to the new default size: Netscape with default body text size set to 20 pixels

You can't set the default body size text with Explorer, though your users can use the Text Size option. Here they choose "Larger".
Choosing "Larger" Text Size in Explorer for Windows

And now the page with sizes set in pixels (left) stays the same, while the page set with ems and percentages (right) grows:
After choosing Larger text size in Explorer

But in Netscape, you can choose larger text as well...Both the text sized with pixels (left) and the text sized with ems and percentages (right) are affected: After choosing Larger text size with default size set at 20 pixels