Most popular search tools:
Start with the DOI of an article, and then choose whether you want the list of references in that article ("citing"), or the list of articles that contain your article as one of their references ("cited by").
As of January 2020, the database contains over 600 million total citations.
It only includes publications that have a DOI assigned.
Technically it can include anything (eg book chapters) with DOIs, but for now it's almost entirely articles.