Why decoupling capacitors used for power rails should be very close to the IC pins?
The ICs contain fast switching transistors generating RF signals. As you know all electrical signals travel in loops. For these ICs the loop is through the power supply pins.

The decoupling caps form a short circuit for these RF signals so the closer you mount the decoupling caps to the power pins of the ICs the smaller the loop will be. This is desirable as it increases the effectiveness of the decoupling because any distance increases parasitic inductance of the wires (about 1 nH per mm). Also large loops emit more RF signals so you have more chance of violating EMI (Electromagnetic Interference) specifications.

Also, a longer distance to the decoupling cap means that the supply voltage inside the IC will be more noisy and polluted with spikes. Worst case the IC stops working because of all the ripple on the supply !

Decoupling caps and bypass caps are indeed the same thing.