difference between core i3,i5 and core i7

I've been googling about it but i couldnt find appropiate answer.long time,i came across an article which i found useful.it is summarized as follows;

intel made core i7 with 4 cores and 8 threads

then made a core i5 removing the hyperthreading and thus cores were 4 while threads were also reduced to 4

then it made an even cheaper processor with hyperthreading and called it ci3 with 2 cores and 4 threads.but removed the turbo boost technology

but now we have corei3,i5 and i7 with 2 cores and with 4 threads.so where's the differnce?.core i5 and i3 can be differentiated because of turbo boost but still there remains no difference between core i5 and core i7. intel atom is also having same no of core/thread ratio as well as dual core processors.so where the difference?i mean core i 3 and core i 5 even have the same size of cache memory..all processors are avaiable in different clock speeds so it cant be the difference that core i7s are available with higher clock speeds than i5s..so where's the difference??