Many databases now offer links to the references cited in papers and to other articles citing the paper.
The Web of Knowledge (a.k.a. The Web of Science) database offers some of the most extensive citation tracing, but it is limited to the journals and books it indexes and may miss some sources (especially some of the more informal sources, like powerpoint presentations, conference papers, etc.)
Google Scholar also provides cited by references.
What differences do you note? Why do you think that is?