Physical styles are actual font attributes such as bold and italic.
Logical styles indicate the meaning of the text and can be configured by browsers to use any typeface and size desired. Some say that it is better to use logical styles than physical styles. However, today's browsers are intelligent enough to interpret physical styles as well as logical styles. For most people, it may be easier to use a bold tag than a strong tag. It doesn't really matter what you choose to use as long as you try to be consistent and that you try to insure that your document looks the way you want it to on as many platforms as possible.