The simple method

\documentclass{article}

\usepackage[T1]{fontenc} %% <- necessary for _ to be displayed correctly
\usepackage{etoolbox}    %% <- for \csuse, \csdef, \ifcsdef and \forcsvlist

%% Define/retrieve a new keyword:
\newcommand\setkeyword[3]{%
  \csdef{keyw@descr@#1}{#2}%
  \csdef{keyw@value@#1}{#3}%
}
\newcommand*\getkeyword[2]{%
  \ifcsdef{keyw@#2@#1}{%         %% <- if the key is defined...
    \csuse{keyw@#2@#1}%          %% <- return the description
  }{%                            %% <- otherwise...
    ??%                          %% <- let's just print ??
  }%
}
\newcommand*\getdescription[1]{\getkeyword{#1}{descr}}
\newcommand*\getvalue[1]{\getkeyword{#1}{value}}

%% Display a table describing a list of keywords:
\newcommand*\keywordtable[1]{%
  \begin{center}
    \begin{tabular}{lrl}
      \textbf{Parameter} & \textbf{Value} & \textbf{Description} \\
      \forcsvlist{\tableentry}{#1} %% <- apply \tableentry to each value in #1
    \end{tabular}
  \end{center}%
}
\newcommand*\tableentry[1]{%
    \formattableentry{\detokenize{#1}}{\getvalue{#1}}{\getdescription{#1}}%
}
\newcommand\formattableentry[3]{ #1 & #2 & #3 \\ }

% %% Declaration of keywords:
\setkeyword{runtype}{A parameter}{1}
\setkeyword{param_1}{Another parameter}{0}
\setkeyword{param_2}{A third parameter}{42}

\begin{document}

\section{Runtype 1 description}
\keywordtable{runtype,param_1}

\section{Runtype 2 description}
\keywordtable{runtype,param_2}

\end{document}