Freeing up space on SD card
It's better if you don't manually delete stuff, unless you can recognize the package names for the apps you want to remove. 
Though it sounds hilarious to install an app to get rid of apps, APPS2SD will neatly list out whatever apps are on the SD card or can be moved between that and the internal memory, so you can make a list of the apps you don't want and manually uninstall them.

Speaking of key folders, here are a couple of common ones-


DCIM - This stores camera photos.
Attachments - Default save location for Gmail attachments.
download - Default file download location.
data - Stores app specific data. If you have games that download additional data, they may store that here.


This is based off my SD card - most of the other folders I've seen are app specific. 

Update - Some phones have a Camera directory, with DCIM under it, so look for both.