is turbulence dangerous for an airplane?

No. Turbulence is usually not dangerous
it is an upset between jet streams

Pilots are skilled enough to avoid turbulent pockets and when you are about to go through them they warn you.
Sometimes it can get very bumpy but RARELY as in almost NEVER
does a plane crash for regular turbulence

The airport officials get turbulence reports before they let flights out and if the turbulence is too dangerous they will not let the flight take off.

The plane flies through the cloud and doesn't hit it.  No, there is not any way it can crash.  Its actually quite beautiful to see the cumulous ( big fluffy ones) clouds and just look out the window and enjoy.

No.

See information about turbulence at http://en.wikipedia.org/wiki/Turbulence

Depends on the severity of the turbulence and the type of aircraft you are flying on.

An airplane going through a cloud will be uneventful except for maybe some bumps of turbulence.

Light or even some moderate turbulence in or around clouds is normal and shouldn't be cause for alarm, especially if you are high above the ground.  In fact, when I fly gliders, I aim for a certain type of cloud because I know that there is rising air fuelling the cloud's growth, and since I want to stay in the air, I aim for the cloud.  What I as a glider pilot would call "lift", airline passengers might call "turbulence".  Its very normal near clouds.

But severe turbulence, for example associated with a thunderstorm (cumulonimbus cloud), can be disasterous, especially if you are low to the ground.  Related to turbulence is wind shear which is a sudden change in direction of winds.  Again, low to the ground, this can be a disaster, so pilots will avoid areas of suspected or reported wind shear to the max extent possible.  In general, pilots won't fly within 20 miles of a thunderstorm cloud because the turbulence can be damaging.  Just make sure you fly with a safe pilot... and of course just about all the commercial pilots out there are safe and need to adhere to strict safety regulation.

Another kind of turbulence caused by other aircraft (wake turbulence) can be dangerous, especially to light aircraft.  this turbulence is caused by wingtip vortices from another aircraft.  These rotating vortexes can flip an airplane upside down!  But no need to worry.  Most pilots don't want that to happen, so pilots and Air Trafiic Control make great strides to ensure there is enough separation between aircraft to avoid it from happenning (the vortices go away after a couple minutes).