I am confused here. What is difference between Phaser Beam Array and Phaser Beam Banks? aren't these the same?

actually no... they are different.

1 - beam banks fire 2 beams, arrays fire 1
2 - beam banks have a 90 degree firing arc, arrays have a 250 degree (i think, feel free to correct) firing arc
3 - beam banks can only be equipped on front weapon slots, arrays have no such restriction
4 - beam banks do more DPS and damage per hit than arrays

they are affected by the same buffs & debuffs

1 - consoles that increase beam damage increase both
2 - captain skills that affect beam weapons affect both
3 - BOff powers that affect beams affect both
4 - feedback pulse hurts u bad

canon-wise...

beam banks are more the original series (kirk's enterprise fire 2 phaser beams)
arrays showed up in TNG. at that time arrays were more powerful and more advantageous tactically. it appears beam banks have made a comeback after being updated.