People volunteer in numerous ways, for numerous reasons. The terminology used to define volunteer roles continually evolves to reflect current trends, such as demographics, economy, and technology. For example, virtual volunteering was an impossible concept until computers and internet became widespread; now, people volunteer virtually all the time, serving communities on their own time, and sometimes creating value half a world away.
Some examples of volunteer categories include: 
These volunteers provide service at regular intervals, such as once per week, often for a long period of time, like a year or more. These volunteer roles often require extensive training or relationship-building, which makes it ineffective for the organization to take on such a volunteer for less than a year. Long-term volunteers may serve in very high-skilled functions, such as palliative care nursing, or low-skill, such as moving furniture. Either way, these volunteers serve as a reliable ongoing presence for the organization. A long-term volunteer position will typically have no specified end date.
These volunteers volunteer for a short period of time, often with a set time period to complete a particular assignment. Many short-term volunteers are mandated to volunteer, such as to obtain high-school credits or fulfill parole requirements; other short-term volunteers have a short window of availability because of life circumstances, such as a work contract which is starting on a particular date.
These volunteers provide a short burst of service, for a single day or a multi-day event such as a music festival. Most of these volunteer positions require minimal training. It is possible to have long-term episodic volunteers who come back every time the event recurs, such as running the registration booth every year at an annual fundraiser. Long-term episodic volunteers may warrant investment such as training.
These volunteers provide service once or more, as it suits their schedule. Timing and task are important to the serendipitous volunteer: it can be useful to define some low-priority tasks which are not time-sensitive, which can be taken serendipitously. Such volunteers tend to be flexible and willing to jump into action if they believe in the organization and its work.
This involved people doing free of charge, the same work they do professionally. Pro bono volunteers may be short-term or ongoing, but generally require a high level of specific skill, such as a lawyer or electrician. This type of volunteer can likely be managed in the same way as a professional consultant hired for a specific task.
These volunteers are cause-oriented, interested in creating new systems and making a big impact. They work well outside of the traditional volunteer framework, without direct supervision, organizing large-scale projects such as advocacy campaigns.
It is worth thinking about which volunteer categories you need when you are developing your job descriptions, so that you can fully communicate the expectations of the role. The better you describe the roles in your job descriptions, the better you can decide who will be best-suited to the role, and the better you can give feedback which will lead to excellence in fulfilling the expectations of the role.
Volunteer Ottawa teaches a number of courses related to volunteer management. Click here for a full listing, and to register.