A genomic library is a collection of clones of small fragments of DNA that together represent the complete genome of an organism. It includes all genes present in the genome, including those that are not expressed. In contrast, a cDNA library constitutes cDNA clones of all the genes expressed in a specific cell type or tissue of an organism.

The genomic library represents the entire DNA content of an organism, where gene sequences are arranged randomly. A major portion of the genome remains untranscribed, and only a small portion is transcribed to give mRNA. The cDNA library, however, represents only the expressed genes, as cDNA is synthesised from mRNA and converted into double-stranded cDNA by DNA polymerase.

An important difference lies in the variability of these libraries. A genomic library of a species remains the same regardless of the tissue source. However, cDNA libraries prepared from different sources of an organism may vary from each other because the expressions of genes are different in different organs or cells at different physiological states.

Regarding applications, the genomic library is helpful for complete sequencing of genome, understanding evolution of species, and comparing sequences of healthy and diseased tissues to identify genetic aberrations. The cDNA library helps identify which genes are active in particular tissues under a particular physiological state and aids in isolating specific genes. cDNA can also be used as probes to screen genomic libraries for a particular gene.