cdna vs genomic library

The first genomic library ever made was for the beta-globin gene taken from human genomic dna of the reticulocytes.
the library was done by using a partial digestion of the genome via restriction enzymes.
[ a partial digest is done by not allowing the reaction to go to completion achieved by limiting incubation time]
the fragments were then transferred to vectors of appropriate size depending on the genome size, such as phage lamda or cosmids . Then colony hybridization techniques were used to screen for colonies that contain the gene of interest using specific and unique probes

cDNA libraries are quite different as their starting material was mrna and not the genome dna.
a cdna library from mrna was made by synthesizing a dna strand complementary to the mran strand called a cdna strand via reverse transcriptase.
the resulting strands were inserted into vectors of choice and the cells cultured and transformed to form colonies.
the colonies were selected for the cdna of interest in a similar way.