In general, all sets are considered to be the subset of big set.

Such a big set of which all other sets under discussion are subsets is called the universal set. 

We normally use the symbol U for the universal sets.

Examples: A set of all  students of a  in a school could be a universal set,

if we consider the different sets of students whose  age is below 10, equal to 10 or below or equal to 12 and above 12.

The set of all numbers below 1000 is universal set,

if we consider the sets of numbers divisible by 2 and 3 and neither by 2 nor by 3 within 1000.