A how-to or a how to is an informal, often short, description of how to accomplish some specific task. A how-to is usually meant to help non-experts, may leave out details that are only important to experts, and may also be greatly simplified from an overall discussion of the topic. One of the earliest How-to books was published in 1519 by Thomas Wight and entitled, A booke of the arte and maner, how to plant and graffe all sortes of trees: With divers other new practise, by one of the Abbey of Saint Vincent in Fraunce by Leonard Mascall.
The correct way to write it in English is "how-to", but it is common practice to write it as "howto" in the Linux community. A possible reason is that this makes it easier to find a how-to in search engines like Google as searching with the words "to" and "how" does not necessarily deliver in relevant search results.