The OCD is a feature for in-system debugging. Although this “emulation” interface lack some of the functionality offered by high end emulators (features like trace/triggers unlimited breakpoints, cycle counters…) it actually has some unique features not found in any other emulator: The most obvious advantage is that you actually are running the code on the device itself, so all electrical and timing characteristics are FOR REAL. No emulated behavior!
avr jtag microcontroller configuration
The main thing to remember is that the JTAG OCD is not an EMULATOR, it is the real thing. Which leads us to the JTAG ICE which actually is no ICE at all. It is a protocol converter/interface allowing AVR Studio to talk to the OCD interface inside the AVR. 
One thing that you should be aware of is that when the AVR OCDEN fuse is programmed (OCD Enabled) some of the clock system is left running even though you put the AVR in sleep mode. So, if you experience high power consumption during SLEEP mode, make sure you are not running the device with the OCDEN programmed!
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