How can clouds store electricity in it?

How can clouds be charged and store the electricity in them?

Clouds don't have electricity stored in them, lightning is static electricity caused by friction that builds up a charge of the opposite polarity of other clouds or the ground. When the charge becomes high enough, it will discharge to the area of the opposing polarity. It's like when you walk across a carpet on a cold windy day with low humidity, the friction between your shoes and the carpet creates a charge. When you reach for the door knob...Zappo!

Start at this page, then click next through the next 6-7 pages. These pages have a very thorough explanation.
http://www.srh.noaa.gov/jetstream/lightning/lightning_intro.htm