From Bioinformatics.Org Wiki
Genomics is a field which existed before the completion of the sequences of genomes, but in the crudest of forms. For example the oft-re-referenced estimate of 100,000 genes in the human genome derived from a(n) (in)famous piece of "back of an envelope" genomics, guessing the weight of chromosomes and the density of the genes they bear. Genomics is any attempt to analyze or compare the entire genetic complement of a species or species (plural). It is, of course, possible to compare genomes by comparing more-or-less representative subsets of genes within genomes.