Get value from the cell above

You can address it like this:

=INDIRECT(ADDRESS(ROW()-1,COLUMN()))

COLUMN() returns a numeric reference to the current column

ROW() returns a numeric reference to the current row.

In the example here, subtracting 1 from the row gives you the previous row. This math can be applied to the ROW() and/or the COLUMN(), but in answering your question, this formula will reference the cell above.

Then we have ADDRESS() which accepts a numeric row and column reference and returns a cell reference as a string.

Finally INDIRECT() allows you to pass a cell reference in as a string, and it processes it as a formula.

Google Spreadsheets give you help hints as you type, so you should get a full explanation of each function as you type the formula above in.

For anyone who stumbles across this question, you can also specify the column by doing something like this:

=INDIRECT("A" & ROW()-1)

This comes in handy if you are returning values in Column B but checking against the previous row in Column A.