If you have an unlimited contract, you can get a blue card which is valid for 4 year (of course you can get permanent residence permit after 33 months). Otherwise it will be limited to your contract period. You will see the company name on your working permit. If you quit, you have 3 months to find a new job. Afterwards, you renew your working permit. So you can change your job but it has to have the same standards. (must be relevant to your education and higher than the salary limit)