35 zone metering splits the image into 35 seperate zones, then uses a proprietary algorithm to try to calculate the optimum exposure based on the illumination on each zone.

Partial metering takes the average illumination intensity in a central area of the image (9% of the total area) and uses that to calculate exposure.

Partial can sometimes give more accurate results if you have a small subject which differs greatly in brightness from the background.

Try this link - http://www.usa.canon.com/content/rebelxt_tutorial/rebelxtlessons.htm and look at lesson #3