If you just want to write between the lines of text, you probably want to use combination of \raisebox and \rlap. \raisebox basically typesets the argument in a box raised, or lowered over the baseline by its given arguments. You however want TeX to make the box of virtually zero width and zero height. This is done using \rlap and arguments to \raisebox respectively. Check the documentation here and also e.g., here.
Now drawing the line from the point you mark and the note is a different matter, I made an attempt with TeX's \rule, but perhaps tikz could help better.
\documentclass{article}
\usepackage{color}
%%% Writes a simple small note above the point of use
\def\note#1{\rlap{\raisebox{.5\baselineskip}[0pt][0pt]{{\footnotesize\color{red}#1}}}}
%%% An attempt to also draw a rectangular line from the anchor to the note
\def\linenote#1{{\color{red}\rlap{\rule{0.5pt}{0.75\baselineskip}\rule[0.75\baselineskip]{5pt}{.5pt}\raisebox{.5\baselineskip}[0pt][0pt]{{\footnotesize #1}}}}}
\begin{document}
Lorem ipsum. Lorem ipsum. Lorem ipsum. Lorem ipsum. Lorem ipsum.
Lorem ipsum. Lorem ipsum. Lorem ipsum. Lorem\note{Some note.} ipsum.
Lorem ipsum. Lorem ipsum. Lorem ipsum. Lorem ipsum. Lorem ipsum. Lorem
ipsum. Lorem ipsum. Lorem ipsum.\linenote{A note with an anchor line.}
Lorem ipsum.
\end{document}
Resulting in the following snippet:

To explain the \note command, it basically writes its argument into a box raised by half the \baselineskip, the spacing between the lines of body text, instructs TeX to treat it as if it was zero height above the box anchor and zero height below the box anchor (the arguments to \raisebox), as well as zero width (the use of \rlap). The latter also outputs the note to the right of the anchor point. If you want to use other alignment, check \clap, or \llap. So far to the position, the text size and color are controled by the standard way.
Finally, you might want to take a look at the package todonotes. It implements something very similar to what you need, except puts the note into the margin, instead between the lines.
texdoc ctex-faqfor the chinese-based faq. you might also search ctan for "chinese" or "cjk" packages. (most of the "ruby" search returns appear to be relevant to the scripting language, though one item does have the caption "Ruby annotations in ConTeXt".) – barbara beeton Aug 03 '12 at 11:58