5) What is “model”? What is “snowball effect”? What is “waterfall diagram”?

Model is a representation fundamental structure of some object or event in the real world. In addition, model means style or design of any item.

References:
http://www.psychstat.missouristate.edu/introbook/sbk04m.htm
http://www.psychstat.missouristate.edu/introbook/sbk04m.htm


Snowball effect means that something begins with small and relatively insignificant and then these things become larger. These things may be harmful after becoming larger.

References:
http://www.urbandictionary.com/define.php?term=snowball+effect
http://en.wikipedia.org/wiki/Snowball_effect


Waterfall diagram is kind of process diagram. This show runs of steps in a progressive environment.
Reference:
http://www.billiondollargraphics.com/waterfalldiagram.html