Journals are categorized by content and by time into volumes and issues. Volume is a temporal categorization on top of issue. Here, "issue" is the number of issues issued in chronological order in a year; and "volume" is the annual number of this publication from the year of its inception in annual order accumulated year by year.

Generally, journals have volume and issue numbers, but there are some journals that have only issue numbers but not volume numbers, and use the year of publication as the volume number. Take Nature and PNAS for example, Nature is two months for a volume, weekly for an issue; PNAS is a year for a volume, weekly for an issue. When we cite the general list of volume and article page number or starting page number, issue number is generally not given out, or if given out is also followed by the volume of the parentheses. For example:

Stouffer, A.L, et al. Structural basis for the function and inhibition of proton channel in SC. Nature,2008,451,596-599.

Stouffer, A.L, et al. Stouffer, A.L, et al. Structural basis for the function and inhibition of proton channel in SC. PNAS,2008,102(05),1596-2599.