What are DNA libraries?

Hint: Normally, a library refers to a large collection of books. Similarly, DNA library means a collection of DNA, gene or cDNA.

Complete answer:
As a term of molecular biology, a ‘library’ refers to a series of fragments of DNA that is preserved and propagated via the molecular cloning process in a population of microorganisms. A selection of DNA sequences from various species is a DNA library or gene library. These DNA sequences are collected, purified, sequenced and connected to a vector in order to be used for analysis, comparative detection, etc. Libraries may have two forms, depending on the source of DNA. These are discussed below:
Genomic Libraries: These are the lists of a single organism's total genomic DNA.
CDNA libraries: This is a list of only the genes that an organism encodes into proteins. It is produced as the starting material using mRNA instead of DNA.
Construction of DNA library-
-DNA is extracted from cells and digested with restriction enzymes.
-DNA fragments are inserted into cloning vectors.
-Bacterial cells are transformed with vectors.

Note:
A genome is the full collection of DNA of an organism, containing all of its genes. Each genome contains all the information necessary for the organism to be developed and maintained. We know that mRNA is produced from a DNA strand during transcription. If another DNA could be prepared using that same mRNA, the strand would contain all the similar bases and sequences. Such a DNA is called cDNA or complementary DNA.