Difficult to answer this in a few sentences! :)
Wind is caused by differentials in air pressure. Air in high pressure zones seeks to escape to lower pressure zones. Due to the rotation of the Earth, the wind is deflected ninety degrees to the right in the Northern Hemisphere. Hence, high pressure in the West and Low pressure in the East would cause a Northerly wind to hit a point further South.
The greater the pressure differential between the two areas of differing pressures, the stronger the wind will be. As the pressure varies due to air displacement and the rotation of the Earth, so too will the speed of the wind.
I hope this very, very simplistic explanation helps...