By using below example of YAML file content will explain about YAML syntax so that easily understandable.

Example :

---
# My personal record
name: Saurabh Kumar Gupta
Title: Sr. Project Lead
skill: JAVA/J2EE
employed: True
domains:
    - Telecom
    - Finance
    - Banking
    - Healthcare
languages:
    ELK: Medium
    JAVA: Expertize
    Scripting: Comfortable
education: |
    MCA
    B.Sc
    Diploma
...

Comments  

YAML documents comments start with #.

Example  : #My personal record

Documents

YAML documents start with (- – -)  and ends with (. . .) (optional)

Example : as above YAML content having only one document.

List

YAML all members of a list are lines beginning at the same indentation level starting with a “-” (a dash and a space).

Example: as above YAML content having domains list as below

domains:
    - Telecom
    - Finance
    - Banking
    - Healthcare

Dictionary

A YAML dictionary is represented in a simple key: value form (the colon must be followed by a space).

Example : as above YAML content having dictionary as languages.

languages:
    ELK: Medium
    JAVA: Expertize
    Scripting: Comfortable

List and Dictionary Together

---
# Employee records
- Saurabh
   name: Saurabh Kumar Gupta
   Title: Sr. Project Lead
   skill: JAVA/J2EE
   employed: True
   domains:
     - Telecom
     - Finance
     - Banking
     - Healthcare
- Gaurav
   name: Gaurav Kumar Gupta
   Title: Project Lead
   skill: ELK
   employed: True
   domains:
     - Telecom
     - Finance
     

Boolean

you can also specify a boolean value (true/false) in several forms.

employed: yes
employed: no
employed: True
employed: false
employed: TRUE

If need to use Boolean value as String Literal use as below

employed: "yes"
employed: "no"
employed: "True"
employed: "false"
employed: "TRUE"

Multiline Value

YAML values can span multiple lines using two ways by  | or >.

•  “Literal Block Scalar” | will include the newlines and any trailing spaces.
• “Folded Block Scalar” > will fold newlines to spaces;

It’s used to make what would otherwise be a very long line easier to read and edit. In either case the indentation will be ignored.

education: |
    MCA
    B.Sc
    Diploma

and

education: '>
     MCA
     B.Sc
     Diploma'

Special Characters

YAML allowed anything to put unquoted but there are some special cases where need to show value with quote.

Character (:) represents as mapping and Character (#) for comments. if these characters are occurs with space in any text value will use with (‘ ‘ or ” “) quotes.

Example :

description: 'you can write your code here: so that we can copy.'
or
description: "you can write your code here: so that we can copy."

The difference between single quotes and double quotes is that in double quotes you can use escapes.

description: "it\'s time to go home."

Variables

YAML uses “{{ var }}” for variables. If a any value after a colon starts with a “{” (curly bracket), YAML will think it is a dictionary, so you must quote it, like so:

Example :

log_file :"{{ LOG_DIR}}\\apps\\app_logs-*.log"

Note:

These reserved or special characters (‘ ” :[] {} > | * & ! % # ` @ ,) can not be used as first character of unquoted scalar. Only allowed characters are ( ? :  ) on beginning of string if a non-space character follows. Better always use quote for these type of scalar values.