Hi Tanu,
Have a look at the
Getting Started Guide to understand the difference between spidering and active scanning.
The is some information about scope in the user guide, which is included with ZAP and also online:
https://code.google.com/p/zaproxy/wiki/HelpStartConceptsScopeYou should only scan/spider "in scope" if you have defined some contexts, otherwise nothing will be in scope and the scans will not do anything.
I think of the 'in scope' operations as being more useful when using ZAP manually, but others may disagree :)
You will typically want to explore you application first, either manually or using the spider.
A report generated at this point will just include alerts found by passive scanning.
You will then probably want to use the active scanner, after which the report will also contain alerts that the active scanner finds.
Does that make sense?
Simon