The simple answer is: all of them. There is probably no other profession that requires such diverse expertise with software tools.
The issue of essential tools for a technical writer is a controversial one. Many employment agencies will only ever discuss technical writing positions in terms of the tools that are involved. Perhaps that is because they do not understand that the real value of a communicator is in facilitating information flow, not mastery of a particular software package. Or perhaps it's just easier to test and rate ability with a particular package.
Unfortunately, many potential employers will also assess your value entirely in terms of the tools you know. Accept this and work with it! You need to know at least one word processor, one graphics package and one online document authoring tool. That will perhaps qualify you as a "graduate".
The one universal tool that you absolutely must know as a technical writer is MicrosoftTM Word. Love it or hate it, you cannot avoid the product. When most employers say "word processor", they mean Word. There are certainly many other worthy products, but none is so ubiquitous.
For documents of more than perhaps 200 pages, many experienced writers feel that Word is too unreliable. The tool of choice in this territory is usually Adobe FrameMaker. Adobe would certainly be miffed to have Frame referred to as a word processor - it is more rightly a layout and publishing package; its word processing capability is in reality inferior to dedicated word processors.
Frame is much more flexible if you plan to implement single-source publishing, with one master document that is published to hard-copy and one or more online formats.
When it comes to graphics, the choices are not so simple. You must decide whether you want to produce vector pictures (where the file contains a description of the shapes and colours that make up the image) or bitmaps (where the file contains explicit information about every pixel in the image).
Vector images are much more versatile and can be easily converted to bitmaps at any time. The files are much smaller and can be scaled without degradation. Bitmaps cannot usually be converted to vector files and do not scale gracefully. For this reason, skill with a vector package is better value. But there is a catch: few employers actually provide a vector package as part of your standard environment. They are much more likely to provide a bitmap package. Often it is the dreaded WindowsTM Paint, or sometimes the JASC PaintShop program. This is perhaps popular because it can be purchased over the internet and is relatively inexpensive. You have two options here: crusade against the ignorant use of a bitmap "paint" program to produce illustrations, or learn to use PaintShop.
If you are fortunate enough to snare a job that has a lot of illustrating, either CorelDraw or Adobe Illustrator are popular. Macromedia Freehand was much loved by career illustrators but was canned by Adobe when they bought Macromedia. It is still widely used but cannot be updated.
Most technical writing positions now require some knowledge of online publishing. This might be for convenient online distribution, or perhaps for use as online help or as part of a web site. The simplest way to cater for this requirement is to use Adobe Acrobat. It can, at the push of a button, produce Web-enabled documents directly from Word. However, there are many compromises in such a process. For example, a page shape and layout that works for hardcopy publishing definitely will not be satisfactory for online publishing. As a minimum, you would need to reformat the document for each medium. If you need to produce a more sophisticated online information product with various types of screens, or that is integrated with an application, Acrobat cannot help.
One serious disadvantage of Acrobat is that users have to download one big file. HTML-based files enable users to download only the files containing relevant information. Acrobat compensates for this by its superior search capability.
Eventually you will probably have to learn to use a help authoring tool (HAT, for those with ADD). Don't be mislead by the silly acronym - they actually produce online information, of which online help is one type. Some integrate with MicrosoftTM Word, although the level of integration varies greatly. The most common model is that you perform a once-only importation of a Word file (e.g. Author-it, RoboHelp Classic, HDK / XDK).
There are also some tools that import FrameMaker files, including Author-it and WebWorks.
From then on, you work with the file only in the help authoring tool. As a Word author, you have littel control over how the online information will look. This is fine if you don't want to work with the Word file after it has been imported into the authoring tool, or the conversion is so simple that you can repeat it at any time.
If you want to keep both the Word file and online version current, you need a tool that can produce help directly from the Word file. RoboHelp Classic, HDK and Word2Web work in this way. With these tools, the Word file persists throughout the project and you can continue to use Word for any changes that are required.
In RoboHelp and HDK, the authoring tool "hijacks" the Word file and inserts special codes, so the help project and the Word file become monogamous partners for life. You can avoid this by re-importing the Word file every time you make changes, but you cannot do this and have access to design features of the authoring tool, like secondary windows, custom toolbar buttons and startup macros. This is definitely a compromise. Would you prefer stand-alone Word files, or a plain-vanilla online information project?
It is possible to produce stand-alone Word files and import them into a help project, and have access to all of the design features of the help project. Only one tool supports this model: ComponentOne's TrueHelp. Word-2-Web is a very easy to use converter than can convert any Word document "on the fly", and without any special markup. It has limited use for online help because it does not fully support topic mapping (matchng a help heading with a part of the software.
Summary In Australia, the most marketable skill is RoboHelp, but Flare is gainng a following. Author-it looked promising for the first few years but has moved beyond the reach of small departments and small businesses due to punitive annual maintenance fees.
Many companies are now investigating XML-based solutions, particularly DITA, but this is a very immature environment and not feasible for a typical company that only produces documentation in support of some other product or service.
This is a term that means different things to different people. Typically, non-expert users assume that it means working with bitmap files, either obtained from clip art collections, or created using tools like PaintShop Pro.
As a technical writer you will likely be asked to use flowcharting packages, presentation packages, paint programs (PaintShop Pro, PhotoShop), and maybe if you are lucky, illustrating packages.
An illustrating package can meet all of your requirements, including production of web graphics. Learning to use one such package is a good investment. The big two are CorelDraw and Adobe Illustrator. Unfortunately many technical communcators expand their skills by first learning a paint package and never get beyond that level. These are actually of limited use for most people, and are not well suited to creation of graphics for print media.
A common problem is that you are expected to produce attractive functional graphics, but no tools are provided. If the need is only a short one, you can perhaps get by with a trial version of one of the popular packages. Many hapless writers are given flowcharting or presentation packages to use for creation of graphics. If you can afford your own laptop, one solution is to bring it and do the graphics using proper illustrating software. Your employer will likely be impressed, never realising that you bypassed their inappropriate instructions to use Visio or Powerpoint. Make sure you also leave the source files in case you (or somebody else) has to make changes.
Special warnng: no matter how desperate you get, never create graphics using Word's built-in drawing tool. Really, it is not worth the maintenance nightmare that you will create. Just don't do it!
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