What is the difference between recessive allele and dominant allele?

The Dominant allele is the allele that determines phenotypical features that manifest in heterozygous or homozygous genotypes.

In the Genetics the dominant allele is represented in uppercase, that is "A", and its recessive allele is written in lowercase, "a".

In the molecular terms generally the recessive allele has a nucleotide sequence previously identical to the corresponding sequence in the dominant allele but that during evolution was inactivated by mutation and this fact describes the expression of the dominant phenotype in heterozygosity (since one functional allele is still present).