I am using Raspberry Pi 2 model B. What is the exact power requirement of this device. Some friends are saying it want 5V 2000mA power supply and some other friends are saying it want 5V 2.5A power supply. In some books I see that without peripherals it will works with 5V 400 to 700mA supply. If I connect 5V 800mA power supply to this device what is going to happen?
There is no simple answer to this question, because it depends on what you want the Pi to do.
I normally run my Pi (B, B+, Pi2, Pi3) from a Apple 1A power supply (it is one of the few I have tested which actually deliver 1A @ 5V), and they work perfectly well with a USB keyboard, mouse and HDMI display. I am sure they would run on 800mA.
When I want to run extra peripherals (such as a HD drive) I use a powered hub. When I use attached projects I use a higher rated supply.
If you want to follow up further Raspberry Pi Power Limitations contains links to various sources.
There is definitely no NEED for a 2.5A supply because the Pi2 has a 2A polyfuse, so CAN"T use more than 2A. (But it won't hurt.)
While you may be able to safely power a Pi on 700mA it is not recommended and leaves little room for expansion (e.g. adding a wifi dongle). You run the risk of undervolting the Pi, which can lead to SD card corruption and other hard to track down gremlins. There is no harm in using a higher amperage charger; the Pi will only draw the current it needs.
In short get the 2.5 or greater Amp charger and a high quality cable and avoid the needless headaches.