1) Word is for writing a business plan to send to venture capitalists, describing the business you want to start.
2) Excel is for doing the financial analysis showing them how your business will be profitable.
3) PowerPoint is for giving the venture capitalists a presentation (think digital slide show) to persuade them to fund your business startup.
4) Access is for storing all your business data, and providing a convenient way to enter and retrieve that data.
They are all very big programs with huge feature sets, and there is some overlap. For example, you can do a table in word, though you can't have formulas in the table cells as you can with Excel. You can do simple data storage in Excel, though if your business is successful you'll probably outgrow Excel's data storage & retrieval abilities and move to Access.
5-7) Answers to these questions would be the size of a large book, but to be brief:
-- Word is mostly about text documents.
-- Excel is mostly about numerical analysis (number crunching).
-- PowerPoint is mostly about digital slide shows.
-- Access is mostly about data entry, storage, and retrieval.
Another way of looking at it is what they generally replaced (and went way beyond) from before computers, though this is a gross oversimplification:
-- Word generally replaced (and went way beyond) typewriters.
-- Excel generally replaced (and went way beyond) calculators.
-- PowerPoint generally replaced (and went way beyond) slide shows.
-- Access generally replaced (and went way beyond) card files.
A good way to get acquainted with these programs and the differences between them is Microsoft's "Getting Started With..." tutorials. Your library probably has these tutorials if you don't want to buy them.
Greg