The ADS maintains three bibliographic databases containing more than 7.1 million records: Astronomy and Astrophysics, Physics, and arXiv e-prints. The main body of data in the ADS consists of bibliographic records, which are searchable through highly customizable query forms, and full-text scans of much of the astronomical literature which can be browsed or searched via the full-text search interface.
Inspec provides access to scientific literature in electrical engineering, electronics, physics, control engineering, information technology, communications, computers, computing, and manufacturing and production engineering.
The database, produced by the Institution of Electrical Engineers, contains over eight million bibliographic records taken from 3,500 scientific and technical journals and 1,500 conference proceedings.
Web of Science is a multidisciplinary database that provides access to current and retrospective information from approximately 8,700 international journals. A unique feature of Web of Science is the ability to find articles that cite a previously published work. Web of Science includes an expanded version of Science Citation Index.
Scopus is a multi-disciplinary abstract and citation database.
Scopus is integrated with, and can simultaneously search Scirus which covers science-related Web pages and institutional repositories. Scopus is able to track cited references: you can use it to search for subsequently published papers that cite an original research paper.