Formulas are made up of several components, as shown in the following example.
Example formula
If([Order Complete]=TRUE, [SUBTOTAL] + [TAX], null)

Formula function:
If()

Argument:
[Order Complete]=TRUE, [SUBTOTAL] + [TAX], null

Field references:
[Order Complete], [SUBTOTAL], and [TAX]

Literals:
2

Operators:
+
Formula function
A formula function is a predefined term that performs an action on values and generates a new value. See the Formula Functions Reference app for a list of formula functions.
You don't need to use formula functions in basic formulas; however, most formulas contain at least one function. Complex formulas may contain many functions. See examples of formula functions.
Argument
An argument is information in a formula function that tells the function which values to act on or produce.

Arguments appear inside parentheses that follow a function

Separate arguments with commas

Arguments can be exact literals, field references, or other functions

Arguments follow an order of operation, so the order you list them is important
Field reference
A field reference retrieves values from a specific field in the record to display or use in a calculation.

Enclose field references in square brackets, e.g.,
[Manager]

A field reference uses a field value in the formula, e.g.,
[First Name] &” “& [Last Name]
joins the values in the First Name and Last Name fields (i.e., “John Smith”) 
Use a field reference to call an application variable; see Creating and using application variables
Literal
A literal is a value that's used exactly as displayed in the formula. Literals can be numbers or text.

Enclose textual literals in double quotes; e.g.,
If([Discount %] > 0.15,"Enter a discount of 15% or less."

Textual literals can contain quotation marks. If a character is part of the literal put a \ before the character; e.g.,
"The \" character is part of this literal.”

Use a backslash to include an open or close square bracket []; e.g.,
“The \[ character is part of this literal.”

To use a backslash in your literal, use a \ before the backslash; e.g.,
“The \” and the \\ are both special characters.”
Operators
Operators are special symbols like +
and *
that act on one or two values to return a new value.
Unary operators
Act on a single value. See list of unary operators.
In a formula, unary operators might look like:

5

+4

not true
Binary operators
Act on two values. See list of binary operators
In a formula, binary operators might look like:

3  4

[Start date] + Days(7)