Literal Value  A text string or number you enter that is not calculated or changed. For example, if you have a value that’s always multiplied by 2% of an amount, your formula would contain the literal value of 2% of that amount:
This example contains every possible part of a formula:  A function called ROUND used to return a number rounded to a specified number of decimal places.
 A field reference called Amount.
 An operator, *, that tells the formula builder to multiply the contents of the Amount field by the literal value, 0.02.
 A literal number, 0.02. Use the decimal value for all percents. To include actual text in your formula, enclose it in quotes.
 The last number 2 in this formula is the input required for the ROUND function that determines the number of decimal places to return.

Field Reference  Reference the value of another custom or standard field using a merge field. The syntax for a merge field is field_name for a standard field or field_name__c for a custom field. The syntax for a merge field on a related object is object_name__r.field_name. Use the Insert Field button or the dropdown list to insert a merge field in your formula where necessary. 
Function  A systemdefined formula that can require input from you and returns a value or values. For example, TODAY() does not require input but returns the current date. The TEXT(value) function requires your percent, number, or currency input and returns text. 
Operator  A symbol that specifies the type of calculation to perform or the order in which to do it. For example, the + symbol specifies two values should be added. The open and close parentheses specify which expressions you want evaluated first.

Comment  An annotation within a formula that begins with a forward slash followed by an asterisk (/*). and concludes with an asterisk followed by a forward slash (*/). For example, /*This is a formula comment*/
Comments are ignored when processing a formula. Comments are useful for explaining specific parts of a formula to anyone viewing the formula definition. For example: AND(
/*competitor field is required, check to see if field is empty */
LEN(Competitor__c) = 0,
/* rule only enforced for ABCD record types */
RecordType.Name = "ABCD Value",
/* checking for any closed status, allows for additional closed picklist values in the future */
CONTAINS(TEXT(StageName), "Closed")
)
You can also use comments to comment out sections of your formula when debugging and checking the syntax to locate errors in the formula. Note
 Nesting comments causes a syntax error. For example, you cannot save a formula that has the following:
 Commenting out a whole formula causes a syntax error.
 Comments count against the character and byte size limits in formulas.
