What is the basic concept of Fourier transform
What information exists in frequency domain?


As JasonR says in the comment, The frequency domain 
is a different view of the same data.  

No new information is created, it just takes the "spatial" domain data (the image pixel values and their locations) and re-presents it as the coefficients of complex exponentials (sines and cosines).


  What is difference between spatial domain and frequency domain?


The spatial domain is the domain of the image: the pixel values are located at particular positions in the image --- they are spatially distributed, usually in a regular grid.

The frequency domain takes this same data and finds any underlying periodicities (sine waves and cosine waves) in the spatial data, and their amplitudes and phases (spatial offsets).

For example, suppose I have the following image (Scialab, not matlab):

N = 100;
x = [1:N];
y = [1:N];
phi = 2*%pi*0.0987298374*ones(N,1)*x + 2*%pi*0.033102974*y'*ones(1,N);
im1 = sin(phi);


Which looks like (appropriately scaled to the grey scale values):



Then the FFT of this is:



(again, with appropriate scaling).

The frequency domain version shows up the periodicities of the spatial domain as a small number of large coefficients.