The effect on electron density in one portion of a molecule due to electron-withdrawing or electron-donating groups elsewhere in the molecule.
 
An inductive effect is an electronic effect due to the polarisation of σ bonds within a molecule or ion. This is typically due to an electronegatvity difference between the atoms at either end of the bond.
						

							
I effect is a permanent effect,and it decrease with increase in distance, consider
C-B-A-X   ,here let X more elecrongegative group  ,so it attracts electrons from the  bond between  A-X and A becomes partially   negative ,similarly A attracts electron cloud   of bond between AB,,but after 3 bonds this effect reduces to a great extent,this is -I effect ,,same as with +I 
						

							
Inductive effect comes in play when there is an electropositive or electronegative atom attatched to the carbon chain. For eg if chlorine is attatched to the carbon chain it attracts the electrons towards itself and a partial positive charge is developed on the carbon.furthur this carbon attracts electron from the next carbon
This effect generally play a role upto 3 carbon atoms.
It is a weaker effect than resonance