Should I trust Google Analytics or Webmaster tool?

Webmaster tools shows only the traffic from Google Search. Analytics includes all other sources also.

I think when we say impressions, it means only for search engines. Am I right???

Please correct me if I am wrong.

Ok. You were talking about impressions. Sorry, I misunderstood it.  

Even I see a huge difference but I have no idea as to why it happens.

Balaji the WMT one is correct one I suppose as it tracks the keywords for which the impressions are reported

But actually, google analytics also tracks the keywords...