A nice overview of the conic sections can be found at this site:

http://www.mathacademy.com/pr/prime/articles/concis/index.asp

This is a great interactive site for graphing conic sections.

In order to use this graphing tool, you need to write the equation in the form

*Ax*

^{2}

*+ Bxy +*Cy

^{2}+ Dx + Ey + F = 0