MBA stands for Master in Business Administration. An MBA is usually taken to enhance your career prospects and build on some of the skills you already have learned in the business world. An MBA could enhance your chances of getting a more senior or managerial position, and as it is at a master's level, it does usually require that you have completed a first degree.
A postgraduate diploma can be obtained in business; this is at a lower level than the MBA. It usually stands for about half the credits you would obtain on a master's program. A diploma does not require the dissertation element, but a postgraduate diploma still requires that you have a first degree.
The difference between a diploma and an MBA is a diploma requires less time and there is no dissertation. An MBA gives you full master's status and is at a higher level. A diploma can be used to step up to a master's level if you feel you cannot do the full MBA now. If you would like to earn the diploma and do the master's program later, your credits can be used on a MBA program when you are ready.
Most MBAs and diploma courses can be studied part-time and can fit around your current working situation. If you are employed in a business setting, your employers may be onboard with allowing you times or days to study. Before embarking on a MBA or a diploma course, it is worth talking to your employer about it and working out how you can manage the workload.