Why "in Proceedings of conference...", but not "in Journal of ...."
You cite proceedings separately from articles contained within them, because proceedings have their own metadata which stands alone, while journal metadata is typically encompassed in the citation of articles.


@article - papers in journals: Chatrychan et al, "Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC", Physics Letters B 716(1) 30--61, 17 September 2012, ISSN 0370-2693.
@proceedings - the proceedings themselves: SIGGRAPH '84 Proceedings of the 11th annual conference on Computer graphics and interactive techniques, July 23-27, 1984, Minneapolis, Minnesota, ISBN:0-89791-138-5 
@inproceedings - papers in proceedings: Cook et al, "Distributed Ray Tracing", in SIGGRAPH '84


You don't tend to cite journals alone (you can just write Physics Letters B, you don't usually need to give a bibliography entry) or journal issues - you rarely want to refer to 'the 17 September 2012 issue of Physics Letters B' instead of a paper inside it.