I want some info about help desk,what is a help desk,how these services shoulb be delivered and so on, who can offer some link about that

Perhaps information found on the following link will be of use to you. I hope that it is.

https://www.askmehelpdesk.com/forum-help/announcement-u-b-click-here-help-using-site-b-u.html

Also, the following link:

https://www.askmehelpdesk.com/announcements/how-do-use-site-14701.html

A helpdesk is a call center that provides assistance with problems. That assitance could range from just phone support to physically have someone come to the location. Beyond that it depends on what they are helping with.