medskipamount, default value in article class

Using \showthe\medskipamount will produce the answer:

6.0pt plus 2.0pt minus 2.0pt.

\medskipamount is a "rubber length", i.e., it can be stretched (plus) or shrunk (minus) by a certain amount. (This is useful for, e.g., stretching page contents so that the last text line will be positioned exactly at the bottom margin.)

\medskipamount is not defined by article.cls, but part of the LaTeX kernel; its definition can be found in section 16.5 "Vertical spacing" of source2e.

Using \showthe\medskipamount will print the quantity in your .log file, just like \show\medskip will print

> \medskip=macro:
->\vspace \medskipamount .

in your .log file, showing what the macro does. For shorthand (rather than viewing the .log file), you can also print the amount in your actual document output using \the\medskipamount.

The printlen package is made especially for printing lengths using \printlength{<len>} within your document and switching between measurement units when printing via \uselengthunit{<unit>}:

\documentclass{article}
\usepackage{printlen}% http://ctan.org/pkg/printlen
\begin{document}
\begin{tabular}{ll}
  \verb!\parskip! in points (\verb!pt!): & \the\parskip \\
  \verb!\textwidth! in inches (\verb!in!): & \uselengthunit{in}\printlength{\textwidth} \\
  \verb!3cm! in points (\verb!pt!): & \uselengthunit{pt}\printlength{\dimexpr3cm} \\
  \verb!2pt+.5\parindent-3bp! in centimetres (\verb!cm!): & \uselengthunit{cm}\printlength{\dimexpr2pt+.5\parindent-3bp}
\end{tabular}
\end{document}

Printing of font-specific components can vary depending on where it's used within the document. For example, \baselineskip is 0pt within tabular, but 12pt within regular text (when using \documentclass{article}).