Only one \baselineskip per paragraph

The \baselineskip is not (as one might hope) a property of a line, but of a paragraph. As a result, in a 10pt (nominal) document (with a default \baselineskip of 12pt), a single character with a larger size, as: 

{\Huge A}
will be squashed into the paragraph: TeX will make sure it doesn't scrape up against the line above, but won't give it "room to breathe", as it does the text at standard size; that is, its size (24.88pt) is taken account of, but its \baselineskip (30pt) isn't. Similarly 
Paragraph text ...
{\footnotesize Extended interjection ...
   ... into the paragraph.}
      ... paragraph continues ...
will look silly, since the 8pt interjection will end up set on the 12pt \baselineskip of the paragraph, rather than its preferred 8.5pt. Finally, something like 
Paragraph text ...
  ... paragraph body ends.
{\footnotesize Short comment on paragraph.}

Next paragraph starts...
will set the body of the first paragraph on the constricted \baselineskip of the \footnotesize comment.

So, how to deal with these problems? The oversized (short) section is typically corrected by a strut: this word comes from movable metal typography, and refers to a spacer that held the boxes (that contained the metal character shapes) apart. Every time you change font size, LaTeX redefines the command \strut to provide the equivalent of a metal-type strut for the size chosen. So for the example above, we would type 

Paragraph text ...
   {\Huge A\strut}
   ... paragraph continues ...
However, more extended insertions (whether of larger or smaller text) are always going to cause problems; while you can strut larger text, ensuring that you strut every line will be tiresome, and there's no such thing as a "negative strut" that pulls the lines together for smaller text.

The only satisfactory way to deal with an extended insertion at a different size is to set it off as a separate paragraph. A satisfactory route to achieving this is the quote environment, which sets its text modestly inset from the enclosing paragraph: 

Paragraph text ...
\begin{quote}
  \footnotesize This is an inset account
  of something relevant to the enclosing
  paragraph...
\end{quote}
... paragraph continues ...
Such quote-bracketing also deals with the problem of a trailing comment on the paragraph.

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=baselinepar