Sometimes the earth and the moon are lined up so that the earth blocks the sun's light from reaching the moon. The moon can not be seen for a short time.
This is called a lunar eclipse. 
Here are some pictures of part of the lunar eclipse on 16/7/2000

• a good site with more information
• http://www.mreclipse.com/Special/SEprimer.html

If, on the other hand, the moon moves into a position where it blocks the sun's rays from reaching earth, the earth is in darkness until the relative postions of the three bodies changes. 
This is called a solar eclipse.
Here is a site about solar eclipses.

• an excellent site

If you find other useful sites, please fill out the form below and send them to me.