Type checking and conversion predicates:

const checks whether a term is a constant
var checks whether a term is a not-instantiated variable
number checks whether a term is a number
integer checks whether the argument is integer or converts it to integer
float checks whether the argument is float or converts it to float
round returns integer which is round the argument
ceiling returns integer which is bigger than the argument
floor returns integer which is smaller than the argument
 

Comparison:

= unify
?= unifyable
\= not unifiable
> bigger than
< less than
=:= equal
>= bigger or equal to
=< less or equal to
=\= not equal
 

Evaluation:

is evaluates righthandside arithmetical expression and unifies the value with the lefthandside variable (Prolog 'assignment')
:= stable assignment
has_to unstable assignment
 

Standard arithmetic:

+ addition
- subtraction
* multiplication
/ division
** exponentiation
// integer division
mod modulo
rem remainder
abs returns the absolute value
sign returns the sign
random generates random numbers
 

Strings:

print converts a term to a string
printq converts a term to a string (without calculating it)
scan converts a string to a term
+ strings concatenation
sub_string finds substrings from the given string
string_end finds ends from the given string
str_length finds the length of the given string
split splits a string using separator character
equal compares strings (with case and no case)
less_than compares strings (with case and no case)
replace replace one string with another
string_from_ASCII makes string from ASCII number
first_ASCII returns the ASCII code of the first symbol
charAt returns a char from its position
 

Char Sets:

char_set gets and sets the default char set
char_set_name returns the name of the char set
convert_charset changes the char set
convert_UTF8_to_local converts from UTF-8 to 8 bit char set
 

Bitwise functions:

>> shift all bits in right
<< shift all bits in left
/\ bitwise and function
\/ bitwise or function
xor bitwise xor function
\ bitwise not function
find_first_1_in Find the first instance of {1} in N
find_last_1_in Find the last instance of {1} in N
find_any_1_in Find all instances of {1} in N
count_all_1s_in count how many instances of {1} are in N
 

Floating point functions:

sin calculates the sine function
cos calculates the cosine function
tg calculates the tangent function
arcsin calculates the arcsine function
arccos calculates the arccosine function
arctg calculates the arctangent function
sinh calculates the hyperbolic sine function
cosh calculates the hyperbolic cosine function
tgh calculates the hyperbolic tangent function
sqrt calculates the square root
log calculates the natural logarithm
log10 calculates the logarithm with base ten
exp calculates the exponent function