The basic function of packet scheduling is to reorder the output queue.The simplest approach being that of having a priority scheme, in which packets with higher priority always leave first.Otherwise Round Robin ,Weighted Fair Queuing can be used.
 

2.2.2 Packet Dropping:-

The controlled dropping of packets is as important as their scheduling. A router drops the packets when it's buffers are full. But at the same time dropping the arriving packet, may cause undesirable behaviour.
In the case of the Internet, with a connection-oriented TCP on a best effort IP service, dropping a packet is taken by TCP as a signal of congestion and causes it to reduce it's load on the network.
Whereas in the case of Real-Time services , dropping more directly relates to achieving the desired QoS. The loss of one packet can contribute to the success of many.