If all the people in the world were to jump simultaneously, what would happen upon landing?
Would there be a earthquake,or something else?

um... not sure... maybe the earths axis would move by a couple milimeters... google.com

Nothing. Nothing at all.

Conservation of momentum and energy should be enough to tell you that.

nothing absolutely nothing. every ant in the world combined is heavier then every person in the world combined.

Not quite. The thing with every human in the world is that that weighs about:
420,000,000,000 kilograms.

The Earth masses in at:
5,900,000,000,000,000,000,000,000 kilograms.

The order of magnitude difference is 13 or so. In other words, humans suck at doing anything to the Earth.

except ruining it.

wasn't this tried, anyway? not that every human participated, but still.

Correction: Ruining the Earth's ecosystem. The worst we've done to the Earth itself is move a little bit of its surface around. I think the deepest we've gone into the surface is around 3 miles. The thing is 40,000 miles deep, so...

the crust = 0.3% of the Earth's mass, and we haven't gotten through more than a single % of that.

simple calculation assuming we all jumped at the same point (note that this is impossible):
F = ma
assuming you accelerate at 9.8m/s^2 downwards:
F = (420x10^9)x9.8 = 4,116,000,000,000n

F = ma
4116*10^9 = (5.9*10^24)*a
4.116/(5.9*10^12) = a
a = 6.97627... x 10^-13

we would cause no impact, and that's in the best possible situation.

Unless if we all comebine to a big round ball as big as a the planet it self and crash to it

I just thwarted that idea, I condensed all humans into a single point of matter, and it still didn't work.