It's a way of easily prototyping using an LPC1768. (or soon.. an M0)
you can use the online compiler and flash the microcontroller very easily. (or there are offline compiler options) then there are software libraries to easily get you up and running with the chip.
the pins of the micronroller are broken out on the board so that you can easily access/connect things to them and it even has some LED's on the board so you can see it "doing things" very quickly.
Most importantly, when you have prototyped with an mBed, you can spin your own LPC1768 board, and use the binary you used on your mbed to run it.
Basically it's a whole solution to get you going QUICKLY.
hey..i am really a beginner in mbed.i came here because my tutor suggest me to use it as it is easier for programming..however i am confuse about what is a mbed now.it looks like the pcb board plus the microcontroller. or is it a system that used to program the micro-controller so i can literally use any type of micro-controller? sorrry about this elementary question..and look forward to your reply.thanks :)