What is a library in the context of recombinant DNA technology?

What is a library in the context of recombinant DNA technology? What are two common types of libraries made from eukaryotic cells? Why are both types of libraries often required?What is a library in the context of recombinant DNA technology?
A library is a collection of DNA molecules from some particular source cloned into a vector. Ideally, a complete library represents every piece of DNA in the original source. The two types of libraries are genomic libraries and cDNA libraries. In a genomic library, genomic DNA is fragmented (either randomly or with a restriction endonuclease) and the fragments are ligated into plasmids or viruses. A collection of those is the library and should contain all of the DNA from the genome. A cDNA library is produced by first isolating mRNA from some cell type, and then using those mRNA to produce complimentary DNA molecules. These cDNA are then cloned into plasmids or other vector. This library would represent all of the expressed genes in the original source, and wouldn't contain either the full gene or any genes that aren't expressed in that particular source tissue. Further, because the cDNA library is produced from mRNA molecules, the library would not contain introns or any of the regulatory DNA from the genome.