GENERATIONS use in laptops (eg: i5 -5th )

The main difference between the different generations is the amount of energy it uses.
 As you go up the generations, they generally use the energy more efficiently, which results in good performance. 
Another thing is performance but intel has a tick, tock method. What this means is that if you go from first to second gen, you will generally not notice it.
 But if you go from first to 3rd you will notice a big jump. I would say that the 4th gen are the best for power users and 5th gen for people that need something with a long battery life. TICK MEANS NEW PROCESS TOCK MEANS NEW Architecture.