At the top of an eps file, you will find the bounding box, a sequence of four numbers which define the left, lower, right and upper edges of the image (in that order). In this masterpiece, the bounding box is 0 0 200 200.
%!PS-Adobe-2.0 EPSF-2.0
%%Title: pumpkin.eps
%%BoundingBox: 0 0 200 200
/tri {newpath 0 0 moveto 10 0 lineto 0 20 lineto -10 0 lineto closepath fill} bind def
1.0 0.6 0.25 setrgbcolor
1 setlinewidth
100 100 100 0 360 arc gsave stroke grestore fill
0 0 0 setrgbcolor
100 90 60 190 350 arc stroke
100 100 translate tri
-50 40 translate tri
100 0 translate tri
showpage
%EOF
Using optional arguments to \includegraphics*, you can define a viewing window, which can be different to the bounding box. The example below displays the upper right corner and the centre of the image.
\documentclass{article}
\usepackage{graphicx}
\begin{document}
\centering
\includegraphics*[100,100][200,200]{pumpkin}
\quad
\includegraphics*[050,050][150,150]{pumpkin}
\end{document}

However, this doesn't work with pdflatex. To get around this, make a copy of the eps file and change the bounding box there before converting to pdf.