My questions are

What exactly is QoS ?

How does It work ?

Does It work on any network or like the RIP service It needs to be enabled on router/server side too ? / Is it client side ?
I have a D-Link 2730U ADSL modem , It has Enable DSCP/TC Mark & Enable 802.1P Mark & Scheduling Strategy (SP , WRR , CAR) what are all these ?

Whats values "Precedence" (also known as priority) can have ?

Is there any free PDF explaining about this ? (free because I can't buy anything on the internet due to restrictions -- I live in Iran)

I really hope this would also help others .

QOS is the term describing the prioritization of certain network services.

In a nutshell, an administrator can assign different levels of importance to network traffic. As an example, VOIP traffic should always be given highest priority while outbound http traffic could be relegated to a lower tier since no one cares if the cat video takes 10 seconds instead of 8 seconds to load.

Precedence is really defined by the equipment you use. But they all use a low-medium-high-highest style of assignment. Even prosumer routers can now be found with QOS settings. My home netgear even allows me to QOS my SIP traffic.

ANy gooogle search will show you dozens of pages explaining QOS. All for free.

QOS can be maintained at either the hardware or software level. Most routers manufactured these days, will have QOS built into the firmware, but most will not allow end-user changes.