What are logical and physical tags in HTML?

In HTML there are both logical tags and physical tags. Logical tags are designed to describe (to the browser) the enclosed text's meaning. Physical tags on the other hand provide specific instructions on how to display the text they enclose.

Physical tags are used to indicate exactly how specific characters are to be formatted.
Like <B> <I> <BIG>
Logical tags are used to indicate to the visually impaired that there is some emphasizes on the text. Each browser has its own technique as to how to indicate to its viewer that the text between the tags are different.
Like <EM> <STRONG>