Consider an array:
{1,2,3,4}
Subarray: contiguous sequence in an array i.e.
{1,2},{1,2,3}
Subsequence: Need not to be contiguous, but maintains order i.e.
{1,2,4}
Subset: Same as subsequence except it has empty set i.e.
{1,3},{}
Given an array/sequence of size n, possible
Subarray = n*(n+1)/2
Subseqeunce = (2^n) -1 (non-empty subsequences)
Subset = 2^n