Analogy means Similarity .
Eg a:b::c:d
Let a:b is LHS of :: & c:d is RHS
then we can say that in Analogy, the relationship between a:b i.e. LHS is same as relationship between c:d i.e. RHS.
Number Analogy: Eg. 12:48::8:32
Letter Analogy: Eg. A:E::I:O
Word Analogy: Eg. Pen : writing:: Book: Reading