You may have seen or heard ISI or Scopus. They are databases that contain a list of journals. Based on some evaluation criteria, the journals get indexed (listed) in these databases.
Depending on your domain of study, either one could be more important than the other. For instance, in science, Scopus finds fewer citations. In social sciences, Scopus finds more citations than ISI.
Comparing ISI and Scopus as a source for citations provides mixed results. In general, Scopus provides a higher citation count than ISI, both in the Sciences and in the Social Sciences and Humanities. In the Sciences, this increase in only marginal (except for Computer Science), while in the Social Sciences and Humanities, this increase is substantial.
Scopus appears to have a much broader journal coverage for the Social Sciences and Humanities than ISI and hence provides a fairer comparison. While in ISI, academics working in the Sciences have, on average, 17.5 times as many citations as the academics working in the Social Sciences and Humanities, in Scopus, this difference is reduced to 7.5 times .
Some universities prefer ISI, and others prefer Scopus. You can refer to your institution to find the details. It is also worth mentioning that there are variations of ISI, for example, SCIE. They are not the same in terms of the number of journals and citations.
Other than Scopus and ISI, there are other publishers like Springer, ACM, IEEE, and many others. However, the most important ones are Scopus and ISI. Other have the same properties as we discussed. They are more active in some research domains.
Both the Scopus and ISI assign DOI to their papers. If you are interested in reading about DOI, check this post.