What is tide? And why there are 2 sides of the same tide at a same
time on the earth?

Dear navig75-ga,

This web site provides a simple explanation,

"Tides are periodic rises and falls of large bodies of water. Tides
are caused by the gravitational interaction between the Earth and the
Moon. The gravitational attraction of the moon causes the oceans to
bulge out in the direction of the moon. Another bulge occurs on the
opposite side, since the Earth is also being pulled toward the moon
(and away from the water on the far side)."

Have a look at the image which helps to show this (the bulge is
greatly exaggerated). There are also explanations as to the effect of
the Sun on the tides.
http://www.enchantedlearning.com/subjects/astronomy/moon/Tides.shtml

Go this web site and on the right is a link titled ?What causes the
tide?. Click on this and there will be an animation which explains in
greater detail how the tides are formed.
http://www.pbs.org/wgbh/nova/venice/tides.html

This site also has an animation to help you understand.
http://www.sfgate.com/getoutside/1996/jun/tides.html

I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.
Thank you
answerfinder

tides moon animation
://www.google.com/search?hl=en&lr=&q=tides+moon+animation&btnG=Search

Dear answerfinder-ga,

I am an Earth Science teacher and I never completely understood why
there was a bulge in the ocean on the other side of the Earth (away
from the moon).  The pbs link and the link to the animation finally
made it clear to me.  It is the centripital force of the earth
rotating around the center of gravity of the earth and the moon!  That
makes sense!  Thank you so much for helping me understand this. 
You'll make me a better teacher.

By the way, I think the rating is unfair because navig75-ga rated you
before he/she could explain how to better your answer (which in my
eyes was very clear).

Also see the information on www.tidclocks.com (http://www.tideclocks.com)