When you employ someone, there will normally be an agreement made about the terms and conditions of that person's employment.
A fixed term contract is precisely what it says it is. The contract defines how long the employee works for the employer.
Most employers do not dismiss any of their employees without good reason, but from time to time it becomes inevitable. The most important document from a legal point of view given to an employee who is to be dismissed is a dismissal letter.
An intern is like an apprentice but instead of someone who is developing trade skills, the intern is developing professional skills.
Long gone are the days when you started work with a company or government department when young and worked with the same employer until you retired. Most people expect to change their job, sometimes their whole career, several times in a working lifetime.