Climate is the average condition of weather at one location over a long duration like a decade or more. Weather is the daily changing status of atmospheric conditions.
We observe the temperature (highest and lowest), atmospheric pressure, cloudiness, rain fall, wind speed, sun shine etc. at one geographical location over 10 years or 20 years or more. Then take a statistical average of that. Then we get the climate and expected weather at that place. Actual weather can be some thing else on a particular day.
Suppose the average temperature in my city was 25 deg.C during the first week of November from 1990 to 2008. If during last 5 years it became 26 deg.C. Then we say that the climatic conditions are changing at my location.
Same way, if average rainfall during 1st week of July was 20 cm during previous decade, and if this decade it is only 18cm, then we say climate is changing.
The reasons for this change are many.
1. Global warming (man made factor)
Due to green house gases etc.
2. Ozone layer depletion.
Ultraviolet rays are not filtered. And they change weather conditions. The amount of radiation received on Earth's surface is changing over years.
3. Eco-system imbalances on Earth
These are the biotic factors. The imbalances in the energy flow cycles and food flow web chains can cause the changes ultimately in the climate.
4. Movement of tectonic plates of Earth's lithosphere
The outermost shell of Earth is divided into tectonic plates. Their large scale movement in hundreds or thousands of years causes changes in climate
5. Presence of volcanoes nearby
The eruptions and emission of hot gases from the volcanoes cause a lot changes in the geological factors that finally affect the climate in that region. These cause gases including sulphur and modify the atmospheric composition.
6. Earth's Orbital variations
There are slow changes in the orbit of revolution around the Sun, in the rotation about itself, and in the axis of tilt. These changes cause the climate changes. Also Earth is not a perfect sphere and that factor adds to the above factors.
7. Solubility of gases in water (oceans)
Various gases (CO2 for example) can dissolve in the ocean waters and cause changes in the composition of atmosphere. That can cause climatic changes.