'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