and the infamous other way...... people just putting deb files on websites to be downloaded...

I think kathy got it dead right though....

Extras-Development - Warning. Only use this area if a) you know what you are doing, b) you are prepared they could damage your device, and even brick them.

Extras-Testing - These should be better for you and are there for testing. Definitely go and check out their current karma, as in most cases the ones nearing 10 karma are very good. A few have almost made it through without being optified. So like me, i've only installed a few - and almost run out of rootfs space. Just be prepared, and check your device after each install.
Just remember - not everyone can always test everything - but anything that may not work should be very fixable by this stage As we all know, Microsoft software is buggy as hell in most cases, but they release updates for people - same scenario here

Extras and Nokia repositories - Should both be safe. This is not to say that software runs differently to the way you expect it too. You can report a bug whenever you want if you thin kit is a bug. Or if unsure, screenshot and describe your issues here to see what others think

(Ctrl+Shift+p to screenshot your device, your image will be saved in the images folder, i then use pixelpipe to share to flickr, and can then show the picture in here )

Advanced search on these forums are your friend - it is most likely someone has posted previous issues or bugs. So remember when you post to give your issue a clear title!
Good Example - UK Vodafone Internet / 3G - File download issues
Bad Example - Why doesn't Vodafone let me download stuff?


Anyway back to the apps! - Extras-Testing always needs extra testers, so please pop in and take a look - start by testing the ones with good'ish karma for safety's sake, and when you get more confident, you can start testing any you want

Good luck!