任何可用于系統篩選和分離特定分子的大的組織化的生物分子的聚合。文庫中生物分子的類型包括遺傳物質如：基因、cDNA、小片段蛋白質肽和小分子化合物。例如，一個cDNA文庫包括單個組織中所有的cDNA,可以篩選出與已知DNA片段雜交的分子。這被稱為cDNA文庫。一個包含可表達為蛋白質的基因的文庫稱為cDNA表達文庫，它可以用來篩選結合特定抗體的蛋白。一個由一系列小分子DNA、RNA、多肽或其它試劑以多種方式組合而成的大量變體的文庫稱為"組合文庫"。同樣，一個以計算機為基礎的藥物結構集合可以用來設計選擇那些小分子，它們的大小正適合插入已知蛋白。這樣的文庫稱為"虛擬文庫"。

Any large, organized collection of biomolecules that can be systematically screened for particular properties, and from which any positive match can be isolated. The types of biomolecules found in libraries include genetic material such as genes or cDNA, proteins in the form of small fragments called peptides, and small molecule compounds. A cDNA library could contain all cDNAs from a single tissue, for example, and could be screened for the ability to hybridize to a fragment of known DNA. This would be referred to as a cDNA library. A library containing the genes that could be expressed as proteins would be referred to as a cDNA expression library, which could be screened for a protein that binds a particular antibody. A library synthesized from a small set of fragments of DNA, RNA, peptides or other reagents that are combined in a variety of ways to generate a large number of variants is known as a “combinatorial library.” Similarly, a computer-based collection of drug structures could be designed to select those molecules of a size that would fit a particular pocket in a known protein. This library would be an example of a “virtual library.”