Can opportunities really help track sales rep performance?

Hello,

I am new to NetSuite and am trying to understand the relationship between opportunity and cash sales. 

For example, when a sales rep creates a new opportunity does the cash sale automatically close the opportunity or does the sales rep have to manually make this change? 

Secondly, what is the easiest way to track a sales rep performance based on the opportunities they claim are won vs. actual cash sales? Is there some way to link actual sales with a specific sales rep?

Thanks,

Amber.