It seems that the concept of an "Opportunity" is one that trips a number of people up and as such I wanted to post some thoughts on this. The core concept of an opportunity is that it is the "thing" that you would consider closed when a sales process is completed.
For instance the opportunity is the tracking of a dollar amount, a close date, a sales stage and a probability percentage. When closed the dollar amount is set in stone, the sales stage is "the end" and the close date is firm and the probability percent is 100. You also have a win/lost concept to consider.
Now for any given Account you can possibly have multiple opportunities. The key here is that as you close these opportunities you are building a historical context. You are also doing analytics and analysis on these open things. You might report on where an opportunity is in the funnel or how likely it is that this opportunity will close.
An "Account" (a client or prospect or other) is ideally a relationship with a company that you maintain forever, but what you are doing with that account might change.
Why would you have multiple Opportunities?
1) Multiple years - Such as a software licensing renewel
2) Multiple projects to close and complete
3) Multiple products to sell to different people
4) Different departments selling different things to different locations