A post-doc, sorry, what? Short for post-doctoral research, is a research job you can get after completing your PhD. It can be anywhere between 3 months and 5 years long, and anywhere in the world too. If you wish to have an academic career, you will usually have to go through the post-doc route, and pick your topics and institutions carefully to increase your experience and improve your CV. More on that if there is a demand…
When you’re a post-doc, you’re a researcher. The difference between you and a lecturer is that you do not have a permanent position (which can be down to many factors, from work experience to country to dumb luck). It is usual to do “a few” post-docs before landing a tenure position or calling it quits.
Even for your first one, just fresh out of your PhD thesis, you will be expected to step up: show independence, initiative, and be able to drive a project forward. It can be daunting, but we’re all human. Mistakes and dead ends are acceptable. But sitting at your desk and waiting that your supervisor hands you tasks is not. Another title for post-doc is research assistant, and it’s useful to think of it that way: Your supervisor does not have the time to do research himself. He’s counting on you to do the good bits and get results. He needs you. And in more than one way, he also envies you!
How many, where, what, with whom etc, are deeply personal questions. Choices can be political (what will look good on my CV to land a lectureship at XX?) or personal (would I really like to live in YY?). The best way to make the best choice is to be honest with yourself and with your objectives.