For those who are newer to machining, you may find the terminology confusing. I first thought tool holders were like belts to hold hand tools. In a machining context, though, tool holders hold a cutting insert in place during machining. The use of these tool holders is only part of the rich world of machining.
What are machine tools? These tools are for holding and manipulating work pieces, usually a rigid metal. Unlike hand tools, these machine tools have several parts that need accurate assembly. CNC machine tools are a step up, as they are automated tools that perform operations thousands of times.
Aside from tool holders, there are other CNC machines as well. Tool presetters measure form tools for dimensions, so they can be made for production runs. Tool posts are parts of the lathe that hold either cutting inserts or tool holders. There are a few other tools, but these are essential to any machinist.
If you are interested in starting a machine shop, see what CNC machines are available to you. Often, tool holders and tool presetters are very expensive, and require some skill. Nevertheless, considering how lucrative American machining is, the results of a well equipped machine shop are often worth it.