In Excel 2002 (XP) and 2003 a new feature called smart tags were introduced.  These appear as small triangles in the corner of the cell. Smart tags can be used for lots of informational purposes, such as stock information, map directions, contact information, etc. They are also used in Excel to show inconsistencies and errors in the underlying data (see below). This tab allows you to customise these tags or disable the recognition of certain types of error.

Under the 'settings' heading you can choose to 'turn them off' by un-checking the box and also change the colour of them from their green default. An option in all Excel error smart tags, is to ignore the error, which effectively removes the marker from a cell(s). You can effectively switch these back on by clicking 'Reset Ignored Errors'.

The Rules section has several rules beneath it, all of which can be independently switched on or off.

Evaluates to error value

Is shown in any cell where the underlying formula, evaluates to one of Excel's error values. #DIV/0!, #NAME!, #N/A, etc.

Text date with 2 digit years

Date contained in the underlying cell, (Formatted as text) with a two digit year which breaks the Y2K rules. You get the option in this tag to convert it to 19xx or 20xx. Obviously this is only for 'text' dates (imported or pre-formatted as text), as any date entered on a sheet is auto-formatted as a date with four digit year, even if entered with two. The 19xx or 20xx being decided by the setting in Windows' regional settings (In XP: Start>Control Panel>Regional and Language Settings>Regional Options>Customize...>Date. The default is between 1930 and 2029, so a date entered as 01/01/30, will be presumed to be 01/01/1930).

Number stored as text

Often this happen unintentionally in imported text, but could also be cells that have numbers but were pre-formatted as text. You will find 'numbers' stored in this way cannot have any calculations performed on them. You can convert these using the option in the smart tags. If you have an older version without smart tags (pre XP) then you can take this route to convert the numbers.

Inconsistent formulas in region

This is triggered when Excel believes that a formula is not consistent with those around it. (In it's current region, which is any block of contiguous data).

Formula omits cells in region

Using the same definition as above for 'region' Excel decides that for what it sees as no logical reason, cells within the range have been excluded from a formula.

Unlocked cells contain formulas

This one shows any cells which are 'unlocked' and contain formulae when the sheet is protected.  One of the major reasons to protect cells is to stop deletion of underlying formulae.

Formulas referring to empty cells

Off by default, when on this would warn of any formulas which referenced empty cells. I suspect this is off by default as there are many occasions when cells reference blank ones before data is entered, e.g. =IF(A1="","",B1/A1)

List data validation error

Is triggered when in lists (XP and up) and a Data Validation rule is broken. It is possible to not have alerts for data validation so with this checked, Excel will display a green triangle if a data validation rule is broken, un-checked it will not.