Can anybody tell me the difference between DCS and PLC's?

PLC is a programmable logic controller specific single processor programmed in ladder logic with minimum of process controls such as temperature control.
DCS is a distributed control system usually a distributed processor but configured in some ladder logic but used mostly for process control items. The configuration is usually in programmed blocks. Someone programming the same programmed blocks in a PLC needs to completely program that functionality.

Today, hardware wise, there is little difference.
IE AB-L5K's are PLC/DCS/PAC just order what you need.
The term now refers more to the industry your in.
I usually find DCS in a power plant, heavy industry with heavy consultants spec's.
or Nuke with crazy validation process's and seperate "super safety stuff".
I usually find PLC running automation and robots in Automotive.
I usually find PAC's in the electronics manufacturing tooling,
( If you call aluminum extruded lego.... tooling !)

I disagree slightly. Today is relates to the question "What is the difference between a boat and a ship?" A PLC is generally smaller than a DCS but can have all the capabilities. I think the difference is mainly who is building the hardware. AB or Siemens, it is a PLC (or PAC), if it is Honeywell or Foxboro, it is a DCS. It is true that a DCS will generally be larger and more complex while a PLC is generally a single machine and less complex. But no clear distention really exists any longer.

@Tom,
Point taken, Each vendor Niche markets so heavily,
The term is starting to reflect the architecture, not the industry.
IE. DELTA V is a DCS .... Omron is a PLC... etc...

A DCS System generally has a high concentration of analog I/O for instrumentation inputs and proportional outputs giving the controller tight control of process variables. The DCS system is often redundant - having dual processors - in the PLC world this redundancy is most often associated with SAFETY PLCs. This is so that PLC errors can be detected and I/O forced to a safe state until the problem can be addressed. Ed made reference to this in his comments about a Power Plant where SIL level 4 is required. Use google to learn more about SIL Levels.

I do agree with Thomas opinion...

In real life the only real distinction between a DCS and PLC is that a DCS can be configured to run mathematical algorithms for special control schemes, while a PLC is limited in the type of control blocks that are available.
Apart from the possibility of these algorithms and the use of programming languages different from ladder logic, the differences are almost non existent.

Thomas absolutely right

Thanks to all of you for giving your valuable comments. These comments helps me a lot.

No.