'string' - literal string value

number - number

NULL - null value

sqrt(a) - square root

sin(a) - sinus of a

cos(a) - cosinus of b

tan(a) - tangens of a

asin(a) - arcussinus of a

acos(a) - arcuscosinus of a

atan(a) - arcustangens of a

to int(a) - convert string a to integer

to real(a) - convert string a to real

to string(a) - convert number a to string

lower(a) - convert string a to lower case

upper(a) - convert string a to upper case

length(a) - length of string a

atan2(y,x) - arcustangens of y/x using the signs of the two arguments to determine the quadrant of the result.

replace(a,replacethis,withthat) - replace replacethis with withthat in string a

regexp_replace(a,replacethis,withthat) - replace the regular expression replacethis with withthat in string a

substr(a,from,len) - len characters of string a starting from from (first character index is 1)

a || b - concatenate strings a and b

$rownum - number current row

$area - area of polygon

$perimeter - perimeter of polygon

$length - length of line

$id - feature id

$x - x coordinate of point

$y - y coordinate of point

xat(n) - x coordinate of nth point of a line (indexes start at 0; negative values apply to the last index)

yat(n) - y coordinate of nth point of a line (indexes start at 0; negative values apply to the last index)

a ^ b - a raised to the power of b

a * b - a multiplied by b

a / b - a divided by b

a + b - a plus b

a - b - a minus b

+a - positive sign

-a - negative value of a

Calculating angles of lines using the field calculator
http://hub.qgis.org/wiki/quantum-gis/Vector_intersection_-_angles