Volatile Functions

A small number of the worksheet functions can be described as "volatile" functions.
This means that these functions constantly update when any changes are made to the worksheet.
If any of these functions are included in your array formulas and they are used with references to large cell ranges then the worksheet will not be very efficient.
Volatile functions will not update when calculation is set to manual.
The following is a list of volatile functions.

CELL	(depends) The number or text string indicating information about a cell.
CHOOSE	(depends) The value in a row (or column) based on an index number.
IF	(depends) The value based on whether a condition is True or False.
INDIRECT	The text string of the contents of a given cell reference.
INFO	The text string returning useful information about the environment.
NOW	The date serial number of the current system date and time.
OFFSET	The cell value which is an offset from a given cell reference.
RAND	The random number between zero and one (>=0 and <1).
RANDARRAY	The array of random numbers between two values (>=min and <max).
RANDBETWEEN	The random integer between two values (>=bottom and <=top).
SUMIF	(depends) The total of the numerical values that satisfies one condition.
TODAY	The serial number representing today's date.

CELL (depends) The number or text string indicating information about a cell.
CHOOSE (depends) The value in a row (or column) based on an index number.
IF (depends) The value based on whether a condition is True or False.
INDIRECT The text string of the contents of a given cell reference.
INFO The text string returning useful information about the environment.
NOW The date serial number of the current system date and time.
OFFSET The cell value which is an offset from a given cell reference.
RAND The random number between zero and one (>=0 and <1).
RANDARRAY The array of random numbers between two values (>=min and <max).
RANDBETWEEN The random integer between two values (>=bottom and <=top).
SUMIF (depends) The total of the numerical values that satisfies one condition.
TODAY The serial number representing today's date.

CELL

This depends on the arguments.
If any of the arguments are volatile (regardless of whether they are used) the function will be considered volatile.

CHOOSE(2,10,RAND())

IF

This depends on the arguments.
If any of the arguments are volatile (regardless of whether they are used) the function will be considered volatile.

IF(1>0,1,RAND())

INFO

This depends on the arguments.

SUMIF

This depends on the arguments.
Becomes volatile when the size of the first range argument is different to the size of the second range argument.

SUMIF(A1:A4,">0",B1) is volatile 
SUMIF(A1:A4,">0",B1:B4") is not volatile

INDEX

This is not volatile despite some documentation from Microsoft that says it is.

ROWS

This is not volatile despite some documentation from Microsoft that says it is.

COLUMNS

This is not volatile despite some documentation from Microsoft that says it is.

AREAS

This is not volatile despite some documentation from Microsoft that says it is.