There is no real answer to that. Excel has hundreds of functions, any of which can be used for business in one way or another. There are a group of financial functions that you would more specifically associate with business, but many other functions can be used by businesses too. Excel is such a versatile application with so many different things it can do, that many functions can be used by business people or others, to do what they require.
Arguments
The SUM function would be the most commonly used function in Excel.
The FV function.
In Excel you can use the ISNUMBER() function.
The NOW function
There is a function called FIND and a function called SEARCH in Excel. There are other functions that can be used to find things, such as the various lookup functions.
Commas are used to separate arguments in Excel functions.
If by the top function, you mean the most commonly used one, that is probably the SUM function.
The SUMIF function.
the asterisk is used for what function when building a formula in excel
In Excel a function within another function would be called a nested function.
You can use the find command of the FIND function.