1> 'each' will loop through each element of the array and evaluate whatever is return inside the block, but it returns the original array without any change.

a = [5,10,15,20]
 => [5, 10, 15, 20] 
a.each {|t| t+2}
 => [5, 10, 15, 20] 
a.each {|t| puts(t+2)}
7
12
17
22
 => [5, 10, 15, 20]

(Thus we see that whatever is written inside the each block is evaluated and the original array is returned at the end)

2>'collect' returns the new array which is created as the result of executing the specified block for every item of the given array.

a = [1,2,3,4]

a.object_id
 => 76272700

a.collect {|x| x*3}
=> [3, 6, 9, 12]

a.object_id
=>76337640

puts a
 => [1, 2, 3, 4]

(Thus we can see collect returns a new array after performing the operations defined in the block on each element of original array, leaving the original array intact)

3> 'map' is similar to 'collect'. There is no specific difference between them in functionality, both of them perform the same thing

c = [1,2,3,4]
 => [1, 2, 3, 4] 
c.object_d
 => 76350610 

c.map {|x| x*5}
 => [5, 10, 15, 20] 

p c.object_id
 => 76350610 

p c
=> [1, 2, 3, 4]

(Thus we can see map also returns a new array after performing the operations defined in the block on each element of original array, leaving the original array intact)