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
http://cs.jsu.edu/mcis/faculty/leathrum/Mathlets/conics.html
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