I have a rather elementary doubt. Some projects like GSM module have heavier current requirements and therefore require to be connected to external power supplies. At the same time I also want to display some data on my computer screen for which I have to connect the USB Cable to the Arduino board, which also carries its own power supply. It is safe for an Arduino board to be connected to external power supply as also power supply through USB cable.
Yes, the Arduino boards are specifically designed for that. Some of the first Arduino boards don't switch easily power, but that was in the past.
You can connect and disconnect the usb-plug and/or external power supply while running.
I don't know which board you have, but I use both external and USB supply simultaneously on the Mega and can see no problem.
From what I see in the boards internals, the internal USB power is automatically switched off if external voltage is applied, thus it looks as if this was taken care of on the Mega (For electronics knowledgeable guys, see http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-sch.pdf, component: IC7B, bottom of page
There is a discussion on the Mega PS here: http://forum.arduino.cc/index.php?topic=22944.10;wap2
Thanks a lot. I will go through necessary documentation