Does a job contract legally oblige you to work?

Perhaps a stupid question, but if I accept an offer for an internship but I get another placement which I find even more attractive, will I be able to back out from the original contract?

The contract is about money usually not the actual labour (i.e. it stipulates what you receive for working between these dates). If you do not work then you do not get paid; the contract becomes redundant. Most contracts stipulate an amount of notice you have to give, but I am unsure whether you could be punished for not adhering to it. I assume an internship would not have a period of notice given the short term nature of the work. But if it did it is unlikely to be more than a week.