Google Docs Help: Google Docs Spreadsheets Function Documentation

Google Docs Spreadsheet Functions Documentation

You can find the official (but not always entirely correct) help when using more>> on the Formulas tab. This will bring you to the overview of all functions and for each function - at the bottom of this window - you can click more>> next to the function plus parameters to go to the help page description. 

The full overview of the functions can be found here (as stated the description is not always correct):

New: http://documents.google.com/support/bin/answer.py?answer=82712&topic=16775 
(Old: http://docs.google.com/support/spreadsheets/bin/static.py?page=functi... )

GoogleFinance function here:

New: http://documents.google.com/support/bin/answer.py?answer=54198 
(Old: http://documents.google.com/support/spreadsheets/bin/answer.py?answer... )

See also: GoogleFinance spreadheet function - how to get historical quotes

GoogleLookUp here:

New: http://documents.google.com/support/bin/answer.py?answer=54199 (Old: http://documents.google.com/support/spreadsheets/bin/answer.py?answer... )

ImportData, ImportXML, IMportHTML and ImportFeed here:

New: http://documents.google.com/support/bin/answer.py?answer=75507 (Old: http://documents.google.com/support/spreadsheets/bin/answer.py?answer... )

ArrayFormula here:

http://documents.google.com/support/bin/answer.py?answer=71291 

For the characters to use in the TEXT function format string please
look here: http://spreadsheets.google.com/ccc?key=pHvTwO20cg0fp0B3YWd06fw

See also GSSFAQ: http://gssfaq.googlepages.com/

How to reference another sheet within the same spreadsheet:

http://documents.google.com/support/bin/answer.py?answer=75943 

How to import data from another spreadsheet:

To get data from spreadsheet A to spreadsheet B:
- Publish spreadsheet A and use the 'More publishing options' to
generate a CSV (comma separated value) type URL for the sheet and
range of cells you want to import on spreadsheet B
- In spreadsheet B use the ImportData function with the CSV-type URL
you just generated, inbetween double quotes, as its single parameter.

See: Functions: Linking data between sheets

See also: Google Docs spreadsheets Import functions speed-up

Using operators in formulas: 

Formulas should always start with an equal sign ( = ) or a plus sign ( + ). E.g. =1+1

Note you can use the usual math operators in formulas in Google Docs spreadsheets:
+   addition, e.g. 1 + 1 --> 2
-    subtraction, e.g. 1 - 2 --> -1
*    multiplication, e.g.  2 * 3 --> 6
/    division, e.g. 9 / 3 --> 3
^    power, e.g. 2 ^ 3 --> 8
you can also compare values, resulting in Boolean values: TRUE and FALSE 
=   equality, e.g. 1=1 --> TRUE
<> inequality, e.g. 1<>2 --> TRUE
<   less than, e.g. 1 < 2 --> TRUE
<= less or equal than, e.g. 3 <= 2 --> FALSE
>  greater than, e.g. 3 > 6 --> FALSE
>= greater or equal than, e.g. 6 >= 7 --> FALSE
Note when a Boolean value (TRUE, FALSE) is part of a math expression, TRUE will be interpreted as and FALSE as . e.g. TRUE + 1 --> 2 ; FALSE * 3 --> 0

So avoid intermixing Boolean expressions and math expressions, or group them with parentheses:
( )  left and right parenthesis, e.g. ( (2*(3+1))/(4-5*7) ) > 0

Finally you can operate on strings
&   concatenation operator, e.g. 1 & 1 --> 11 ; "Hello"&" "&"world"&CHAR(33) --> Hello World!

Examples:
A1: 10
A2: 110
A3: = (A1+A2 * A2-A1 ^ A1) <> (2 ^ -3 + A2 ^ ( 1/3) * 4)

Using cell references:

A cell reference is made by putting together a column indicator (one or two letters: A .. IV) and a row indicator (an integer number 1 .. 10000), e.g. A1, B4, AA12, IV10000 .

A cell reference can be used in a formula, e.g.

=A1+1  

=INT(A1/1000)

=( A1 ) = ( MOD(A1,B1)+INT(A1/B1)*B1 ) 

When using Copy Down/Right, AutoFill and Copy & Paste these cell refences will automatically change depending on the row/column they are copied to; for this reason we call these relative cell references.

Cell references can be fixed, that is only the column, only the row, or both column and row part can be made unchangeable by Copy Down/Right, AutoFill and Copy & Paste. This is done by prepending the column indicator, the row indicator or both by a dollar sign ( $ ); these are called (partially) absolute cell references.

Examples: