Arubaito is a gairaigo (foreign loanword) rather than a native Japanese term. It is derived from the German word for work, arbeit. Arubaito refers mostly to traditional part-time labor, but it can also refer to moonlighting, or even working as a “temp” when more stable employment is not available.
In Japan, as in the U.S., students take part-time jobs to help cover their educational expenses and other sundry purchases. However, arubaito during the school years is generally viewed as a distraction to the more important job of studying--especially when school entrance exams are looming.