Technical writing bridges technology to the people that use it. The function of a techical writer is to, clearly and succinctly, let people understand how a piece of technology works and how to use it to suit their purposes. These days, 'technology' might be anything from a complex multi-site computer-driven automation system to a plain container needing to be assembled out of the box. Both would require a manual or guide produced by a technical writer.
I've worked as a technical writer for more than 20 years for corporations and businesses large and small, producing technical documents large and small.
Large corporations I've worked for include Hughes Aircraft and North American Rockwell (aerospace), Douglas Lighting Controls and Rheem Califone (electronics) and Cubicomp/Wavefront (computer software). Those jobs all involved the writing of many large and complex documents of a highly technical nature.
For both large and small businesess I've written a lot of material (assembly instructions, for instance) which needs to be brief -but clear and thorough- in showing the user how to get to set up and use the product.
My technical education and background allows me to understand technical concepts readily. My writing background, augmented by my illustration background, allows me to present them clearly and direct them to the target audience via print or online. As well, I can edit and update technical documents written by others for style, accuracy and clarity.