The term workforce is useful when you talk about a group of people who work or are available to work. You might, for example, talk about the information technology workforce or the workforce in Austin, Texas.
When you use the noun workforce, you are always talking about a lot of people, and you're usually referring to everyone in a particular industry or area. Economists often discuss the entire country's workforce, and you've probably heard about the auto industry workforce or the nursing workforce on the news. Workforce can be a singular or plural word, since it's used for a group of many individuals. It's been in use since the early 1960's.