What is important in a job description? I am applying for a masters programme in Computer Science in the UK.
In the application I am asked to provide a description of any jobs I had so far. 

What is important in such a job description, i.e. what information should I include? 
How long should it be?

There is an html form with a text field but the maximum length is unspecified.
If I was on a graduate committee, I would want to know


How long you held the job. 
What your responsibilities were: e.g. if I was an aide in an undergrad class, did I grade homework? Did I teach the class sometimes? Did I have contact with students? Did I write tests?
How did this job prepare you for admittance into the graduate program. What relevant skills did you obtain?
If the person that oversaw you in the said job is known to the application committee, perhaps say who you worked for. Or if it is a company, say who the company is. There is a huge difference between "Software Engineer" at a company with 5000 employees and "Software Engineer" at a start-up in someone's basement.