There are many different systems for the citation of references. The most common systems are the name-year system (the Harvard system), the number reference system (the Vancouver system) and the footnote reference system. These are also accepted in the standards SFS 5989, ISO 690:1987 and 690-2:1997, concerning text and bibliographic references.
Many disciplines have their established practices to cite sources, and citation styles and techniques vary between scientific fields. The most important thing is to be consistent in the way you record your references. Use always the same style in your in-text citations and bibliographic references in the same publication.