Help:Introduction to Categories
Categories are an way to tag articles in a way which makes them accessible and well-arranged. To see a list of existing categories, click here.
To add an article to a category, simply put [[Category: Category name]] in an article. (Convention dictates that this be at the very bottom of the article, but in principle it could be anywhere.)
This creates the category if it did not already exist. However, since a new category is empty of any descriptive text, it will still appear as a red link until there is text in the category's header (which you can edit as you would a regular article).
To arrange a sub-category of a category, simply edit the sub-category's page to place the sub-category in the parent category, just as you would to place an article in a category. The sub-category will automatically be placed in the parent category.
To link to a category without placing the article in this category, prepend a colon to the link, like this: [[:Category: Category name]].
To avoid needing to tag an article with many categories, we are attempting to organize the categories to mostly be in a hierarchy, and only tagging articles with the most restricted category. That will automatically make them available through the higher-level categories as well.
For instance, the category Category: Memories contains the sub-category Category: DEC Memories, which then contains the sub-sub-categories Category: UNIBUS Memories, Category: QBUS Memories, etc. Tagging an article with just one of the latter automatically enables it to be found via any of the more encompassing categories.
Some articles may need to be placed higher in the hierarchy than other seemingly-similar ones, if there is no appropriate sub-category; e.g. Multics has to go in the top-level category Category: Operating Systems since it was sui generis.
Sub-categories themselves may also appear in more than one super-category; e.g. Category: IBM Mainframes appears in both Category: IBM Computers and Category: Mainframes. This allows all the articles in the IBM Mainframe category to only need a single category tag, to be reachable via either the IBM Computers or Mainframes super-category.
Here are a few details about the current category organization for the more filled-out areas of the Wiki, e.g. DEC computers (which is used here as an example solely because it is currently the most complete).
At the top are a set of 'meta-categories', all included in an overall category, Category: Digital Equipment Corporation (which is itself in the top-level category Category: Manufacturers): Category: DEC Architectures, Category: DEC Buses, Category: DEC Memories, Category: DEC Processors, Category: DEC Systems, etc. (These are themselves generally also included in wiki-wide 'super-categories', e.g. Category: Architectures, Category: Buses, etc.)
Beneath this are categories, similarly divided, for each line: e.g. Category: PDP-10 Processors, Category: PDP-11 Processors, etc; these are included in the appropriate DEC-wide category. (The difference between the 'Systems' categories and the 'Processor' categories is that the latter applies only to the CPUs, whereas the former includes complete systems, including main memory, disks, etc.)
There is also a category for each line, which includes everything to do with that line; it includes the per-line categories for that line, and also articles about the line which don't fit any of those categories. See, for example, Category: PDP-11s.
Finally, in some cases, there is additional complexity.
E.g. since there are many OSs for DEC machines, but which were not written by DEC, there is a further category for these 'non-DEC OSs': Category: Non-DEC Operating Systems. (This is in addition to Category: DEC Operating Systems, for those written by DEC; and per-line categories - e.g. Category: PDP-8 Operating Systems, Category: PDP-10 Operating Systems, Category: PDP-11 Operating Systems, etc.) This category allows all these 'non-DEC operating systems for DEC machines' to be found easily. Articles on OSs for DEC machines will thus be tagged with two categories; one for the line, and one for their DEC/non-DEC origins.
There is no fixed answer as to whether it is better to create a sub-category which can be included in two super-categories (as with the IBM mainframe example), or instead include articles directly in two sub-categories (as with the OSs for DEC machines). The 'two-tags' route was taken in latter case to prevent the creation of too many very small categories. (See the discussion about this case here for guidance for similar cases.)