Primer: Journal Indexing
Journal indexing refers to the practice of calculating performance metrics for scholarly journals. Calculating such metrics reliably requires access to a large number of publications and is thus often offered as a secondary service by influential publishers and research institutions. In an effort to provide high-quality metrics indexing services will often be selective regarding the journals they will calculate metrics for/from. Such journals are said to be "indexed".
The History of Journal Indexing
Before the modern internet age finding resources in a large library could be a time intensive process. To make this process easier libraries would create indexes of their collections so they could be found according to selected criteria (e.g., author, title or subject). Today, the most visible outcome of this period is the Dewey decimal system, which serves as a kind of location based index.
In time scholars began to discuss what kinds of indexes were needed to support high-quality research. It was under this topic that Eugene Garfield published his famous 1955 article Citation Indexes for Science . Garfield proposed that a new index should be created to help scholars find publications that were related according to citations. Garfield argued that such an index would make it easier for scholars to find valid but little known criticisms of well-known research. In addition a secondary benefit of such an index would be the potential to calculate an "impact factor" for specific works and authors.
In 1960 Garfield founded the Institute for Scientific Information (ISI) to create his proposed Citation Index . The ISI created three indexes: Science Citation Index, Social Sciences Citation Index and the Arts and Humanities Citation Index. In addition, the ISI also annually published the impact factors for all the journals it indexed. Thus the ISI was the first modern journal indexing service. Over time the ISI would pass through several hands and its indexing services would become less influential while its impact factor would grow in influence eventually leading to modern times.
Modern Journal Indexing Services
In present day there are many journal indexing services that compete for influence. Below are a few examples:
- Journal Citation Reports
- proprietary metric is the impact factor
- formerly owned by Thompson Reuters
- metrics for/from proprietary list of curated journals
- proprietary metric is cite score*
- owned by Elsevier
- metrics for/from propriety list of curated journals
- Scimago Journal and Country Rank
- proprietary metric is SJR
- owned by a research group at the University of Granada
- metrics for/from Scopus's list of curated journals
- Google Scholar Metrics
- proprietary metric is h5-index
- owned by Google
- metrics for/from uncurated list of journals
Modern Journal Indexing Metrics
Due to the development of the internet and search engines most journal indexing services no longer provide citation indexes. Rather, their main purpose is to provide performance metrics for journals. Every service, in an effort to differentiate itself, provides its own "flagship" metric. These metrics, despite their long history and influence on the research community, remain highly controversial  and an area of active research.
 Garfield, Eugene. "Citation indexes for science: A new dimension in documentation through association of ideas." Science 122.3159 (1955): 108-111.
 Vanclay, Jerome K. "Impact factor: outdated artefact or stepping-stone to journal certification?." Scientometrics 92.2 (2012): 211-238.