i am new to this !!
i want to make a octacopter !!
first i started with brushed motors but they did not provide me sufficient thrust !!
so i am thinking to move to brushless motors !!
but i am not able to understand how they work !!
till now what i have understood is that brushless motor is some what like a 3 phase induction motor
so to produce ac a esc is used ...
but i am not able to understand how to power the esc !!
can they be directly connected to the battery or we need some microcontroller to controll it !!
also i want to know what is this 30 amp or 25 amp written on the esc !! is this linked to the motor or to the amount of current being supplied !!
THANX IN ADVANCE 

ESC looks like this:


          #############
+  ----#                      #-------------- A
-  ----#                      #---------------B
         #                      #---------------C
+ ----#         E.S.C      #
- ----#                       #
S ----#                       #
        #############

The top +, - go to the Lipo/NiCd/Battery source.
The bottom +, -, S go to the receiver (Rx). Here +, - are regulated power supply for your Rx. The receiver controls the output (A,B,C) that goes to the motor based on the signal(S)
The A, B, C are the three leads that will connect to the 3 wire brushless motor.

All motors are loads in an electric circuit. So every motor draws some current. The amount of current is based on the load on the motor itself (the propeller etc). 25A on an ESC indicates that it can safely take 25 A of a load without burning out. If the motor draws more than 25A, the ESC will burn out, so 25 A, 30 A etc are upper limits of current that the electric motor can draw...

Hope that helps

k k !!
but if i want to make a wired octacopter then what would i do to the +,-, and s ?? (talking about the leads which go to the receiver!!

The + - and S go into a FLIGHT CONTROLLER, not to the Rx (receiver) directly.
For today's quads/hexas/octas:

Receiver --------> Flight controller -----------> ESC -----------> Motor(s)
                                                                    ^
                                                                    ^
                                                                Battery (usually a LiPo)

Vineet

Never worked with multi rotors per se, but my assumption would be that the +, -, S leads will plug onto the octocopter controller board (which like you presumed would have a mup to generate the mixing of the motors). 8 ESC's plugged to motors on one end (A,B,C) and to the controller board on the other (+, -, S).

Will leave it to the rotor guys to answer...

Which FC do you want to use anyway?

Please have a look at this guide. It might help you to understand some basics of motor, esc, battery, fc and props
http://www.rcindia.org/multirotors/tutorial-how-to-build-multirotor-from-scratch/