LaTeX Error: Environment keywords undefined

Define your own \keywords macro:

\documentclass{article}
\title{My title}
\author{My author}
\providecommand{\keywords}[1]{\textbf{\textit{Index terms---}} #1}
\begin{document}
\maketitle
\begin{abstract}
Lorem ipsum\ldots
\end{abstract}
\keywords{one, two, three, four}

\section{Introduction}
\end{document}

I use \providecommand instead of \newcommand to accommodate the possibility that the document class you're using has defined \keywords already. \providecommand will only define it if not already defined.

\listfiles
\documentclass{article}
\usepackage{spconf}

\begin{document}
foo
%
\begin{keywords}
One, two, three, four, five
\end{keywords}

\end{document}

my files from the logfile:

 *File List*
 article.cls    2007/10/19 v1.4h Standard LaTeX document class
  size10.clo    2007/10/19 v1.4h Standard LaTeX file (size option)
  spconf.sty    
  ot1ptm.fd    2001/06/04 font definitions for OT1/ptm.
 ***********