The Main Difference Between PLC and DCS
Programmable Logic Controllers (PLC)
Programmable Logic Controllers (PLC) were first created to serve the automobile industry, and the first programmable logic controller project was developed in 1968 for General Motors to replace hard-wired relay systems with an electronic controller.
Programmable Logic Controllers, are frequently used to synchronize the flow of inputs from sensors (Physical) and events with the flow of outputs to actuators and events. This leads to precisely controlled actions that permit a tight control of almost any industrial process.
This is actually a control device that consists of a programmable microprocessor, and is programmed using a specialized computer language. 
Before, a programmable logic controller would have been programmed in ladder logic, which is similar to a schematic of relay logic. A modern programmabl logic controller is usually programmed in any one of several languages, ranging from ladder logic to Basic or C. Typically, the program is written in a development environment on a personal computer (PC), and then is downloaded onto the programmable logic controller directly through a cable connection. The program is stored in the programmable logic controller in non-volatile memory.
Programmable logic controllers contain a variable number of Input/Output (I/O) ports, and are typically Reduced Instruction Set Computer (RISC) based. They are designed for real-time use, and often must withstand harsh environments on the shop floor. The programmable logic controller circuitry monitors the status of multiple sensor inputs, which control output actuators, which may be things like motor starters, solenoids, lights and displays, or valves.
Digital signals yield an on or off signal, which the programmable logic controller sees as Boolean values. Analog signals may also be used, from devices such as volume controls, and these analog signals are seen by the programmable logic controller as floating point values.
There are several different types of interfaces that are used when people need to interact with the programmable logic controller to configure it or work with it. This may take the form of simple lights or switches or text displays, or for more complex systems, a computer of Web interface on a computer running a Supervisory Control and Data Acquisition (SCADA) system.
Distributed Control System (DCS)
Distributed Control System (DCS) is a system of dividing plant or process control 
into several areas of responsibility, each managed by its own controller, with the whole system connected to form a single entity, usually by means of communication buses.
Distributed Control System (DCS) refers to a control system usually of a manufacturing system, process or any kind of dynamic system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system controlled by one or more controllers. The entire system of controllers is connected by networks for communication and monitoring.
A DCS typically uses custom designed processors as controllers and uses both proprietary interconnections and Communications protocol for communication. Input & output modules form component parts of the DCS. The processor receives information from input modules and sends information to output modules. The input modules receive information from input instruments in the process (a.k.a. field) and transmit instructions to the output instruments in the field. Computer buses or electrical buses connect the processor and modules through multiplexer or demultiplexers. Buses also connect the distributed controllers with the central controller and finally to the Human-Machine Interface (HMI) or control consoles
What is the difference between PLC and DCS?
The main difference between Programmable Logic Controller (PLC) and Distributed Control System (DCS) are:
The number of I/O in the system. DCS is dealing with very large no. of I/O with less cost. also DCS comes with it's SCADA always. if DCS stopped, all system is stopped. if the system has less No. of I/O, then PLC is better from cost view. Also SCADA is optional. if system is controlled by no of PLC. if PLC is down. others continue working Normally. Historical, main difference was that DCS deal with Analogue signals but PLC deal with Discrete digital system. Now DCS has Digital Modules and PLC has Analogue modules.
Besides, PLC has a processor and input and output cards (I/O card could be digital and/ or analog) Processor has the software - basis the input it receives from field devices it gives out the output commands to control devices of the field. DCS is a group of individual smart controllers (again having embedded software and connections with field devices) doing their specified operation (duty) but interconnected.