First a disclaimer: If you want legal advice then you need to consult a lawyer. I am not one and below if what I have gathered over the years and share to try and be helpful, but it is not guaranteed to be correct.

First the commercial/stock photo situation. For this you will always need a model release to be filled out and signed by the model, or their guardian if under 18.

Stock sites/agencies will insist on this for every person in every photo. So you will need this explicitly for each shot that you upload. This also includes self-portraits!

There is an exception, if the photo can be considered reportage (for editorial use only). But then the use of it is different.

An example of what one site requires is here: http://www.istockphoto.com/article_view.php?ID=648

In general, it is the publishing of the photo and not the taking that is important. So you can take any photos you want and they can't stop you. However, if they do complain to the police then you will probably still be asked to delete the photos, so this is perhaps not so clear. And if taking photos on private property then you should first get permission.

For non-stock photos then you need to consider the domain that you are publishing in. If they are published in a private domain, then you don't normally need any permission. So for example you could publish on your own web site but only provide access to specific limited people (making people register does not count!).

Otherwise you are publishing them publicly, and here you need to be more careful.

If the shot is of a single person then you will need their permission. If it is of more than 1 person, should as a group or in a crowd scene then it is not so bad. But the exact legal position here I am not sure about. Note that if you zoom in on somebody in a crowd that this is then an individual shot, not a crowd scene.

Also, to take photos a specific events you will normally need permission (sports events, concerts etc.)

People in the public eye are different (Personen des Zeitgeschehens), and you can photograph them without permission but only if the photos are not embarrassing. But be careful, as any photos of minors associated with these people are not allowed.

All of this only applies if the person is recognisable in the picture. So if they have there back to you for example then it will probably be OK.

Some information in English is here: http://guidedmunich.com/german-privacy-laws/

In general, you should always ask and get permission. And be prepared to take photos down if asked (and provide an easy way to allow this).