In roster form, all elements of set are listed.

Example:

###
**
Set of natural numbers less than 6
**

Natural numbers = 1, 2, 3, 4, 5, 6, 7, 8, …

Natural numbers less than 6 = 1, 2, 3, 4, 5

So, set is

A = {1, 2, 3, 4, 5}

Note that, in set, all elements are listed

###
**
Set of vowels in English
**

Vowels are a, e, i, o, u

So, set is

B = {1, 2, 3, 4, 5}