What is a Citation Manager?
Citation managers allow you to save all of your citations in one place. They also help you create bibliographies and in-text citations. Always double-check citations from citation managers. They sometimes make errors in formatting citations, and you may have to do some manual edits.
Which is best? That completely depends on your needs and preferences. They generally offer the same general functionality, but with different interfaces --- and with different "bells & whistles".
Some web sites allow users to enter the citation information (author, title, journal, volume, etc.) and will create a citation for you on the fly which you can copy and paste into your bibliography. Always double-check citations from citation generators. They sometimes make errors in formatting citations, and you may have to do some manual edits.