Today I want to take a slight detour from the normal SEO stuff, this is still very much related to SEO as you will see towards the end but I'm going to start with using EM as a measure before moving onto EM for emphasis. The EM as a measure of size is something that appear to confuse most designers and developers, so I want to simplify this today with a few examples.
1em is related to the font size of any given page, div or table. So if your default font-size is 12px and you never overwrite this in a div (so it is inherit to all areas) then 1em in all areas will be the same size as 12px - change default font-size to 100px and 1 em is now 100px ... its fairly simple. One thing to remember is that an em is measure to the maximum externals of any give font-family.
Now we've done that little explanation and cleared that up I want to explain another scripting EM.
This EM is one you find within the content coding, it stands for emphasis. From that you can probably gather it is to apparently emphasise a point you are making, though more often than not it is just used to create italics within content - given this is its default styling (though as with all html content tags you can style this as you wish within your CSS so it can simply increase the font-size or colour of the tagged content).
Some search engines use this tag to find out your key content points, the larger one's apparently don't read the tagged content as being more important than the non-tagged content. But all search engines take note of your heading tags... so the point I want to make is this:
Your headings should be your main contextual points
Sounds fairly simple right? so hop to it and improve your content.