How does the Sun affect our climate?

The Sun drives the weather on planet Earth. The winds and circulation of ocean patterns are all affected by the Sun's energy output. The differential heating of the planet, due to the tilt of the rotation axis of the Earth with respect to the Sun, generates the winds and major ocean currents as well as providing us with our seasons. Furthermore, it is believed that the 11-year solar cycle has an impact on our climate. 

An excellent review of the climatic impact of the Sun on Earth can be found at the Space Weather Prediction Center (NOAA).