I work in academia. The way contracts work (in most of the USA) is that you are on a 9 month contract and have the summer free with no salary. This means that you officially have not duties in the summer (but in practice it may be different), but you receive no salary for those months. In theory you are expected to fund your summer by teaching or getting someone to pay for your research (NSF, NIH, NASA, etc). In practice, there are so many professors and so little summer funding that most do the research for free.
How can I briefly explain that I am not working in the summer because I am not paid? For example, say that I decide to just do without the money and not work (and instead take a vacation or do volunteer work). Is it misleading to say even though I am a professor I am not working in the summer because I am not paid? Some colleagues reply that they are being paid, since you can request your 9 month salary to be spread in 12 months, but that is misleading (what would you say if you are paid 3/4 of your annual salary and told you have summer free?). Other colleagues reply that you are in essence unemployed in the summer.
NOTE: If I were to take a 12-month job, I would earn more than I am making now (roughly 3 months more of salary).