You can pull data from other sources, whether or worksheets in the same spreadsheet or from other spreadsheets.
Example 1
Below is an example that shows the addition of values in a ange of cells on a specific worksheet in a separate file
Cell A10 contains the following formula:
=SUM('[Data2000.xls]DataSummary'!$B$30:$B$60)
In this example, Data2000.xls is the file name and DataSummary is a specific worksheet in that file. The data pulled is from cells B30 through B60
Example 2
If you want to display specific data from a cell in another worksheet or from another file, it could like this
Cell B2 has the following formula:
=T('[Data2000.xls]DataSummary'!$C$29)
In this example, the cell is pulling a string of text in cell C29 in the worksheet DataSummary which is located in a file called Data2000.xls and displaying that text. For example C29 in Data2000 could be "February"... so in your spreadsheet, B2 is disaplying "February".
To get these functions...
You have multiple options
Highlight the cell in which you want the info displayed.
Select fromt he pull down menu Insert-->Function
A window appears with lists of numerous functions. Hilighting a function gives a brief explanation of the function. The help within Excel will describe each function in more detail.
You will then be directed to provide data or location to source data. You will be able to switch to other worksheets or even other spreadsheets and select a cell or a range of cells.
That's the basics.
Alternatively, once the display cell is hilighted you can select the = symbol in your display bar and a drop down list of functions will appear.
You could even place a button on your toolbar for calling up the functions list.