What are they and what are they for?

1XX - Informational

Indicates only one information. Examples:

100 - Continue.
102 - Processing

2XX - Success

Indicates successful request. Examples:

200 - OK
201 - Created
202 - Accepted

3XX - Redirection

Indicates a redial. Examples:

301 - Moved Permanently
308 - Permanent Redirect

4XX - Client Error

Indicates an error on the client side. An example that we all face is when we enter the URL incorrectly. Examples:

400 - Bad Request
401 - Unauthorized
403 - Forbidden
404 - Not found

5XX - Server Error

Indicator an error on the server side. Examples:

500 - Internal Server Error
502 Bad Gateway
503 - Service Unavailable