The tasks people actually search for, answered as they stand on flickr.com first. The app is named where it saves the work, and not where it does not.
A Flickr API key is issued by Flickr, free of charge, from the App Garden at flickr.com/services/apps/create. You need a Flickr Pro account to create one, the process takes about five minutes, and you get two values back: a key and a secret. Any third-party app that talks to Flickr on your behalf, including Photo Stream Assistant, needs both.
There are three ways to download everything you have on Flickr, and they differ in what you get back. Flickr's own data export is the fastest first copy and comes by email in zip files. The website downloads selections of up to 500 items at a time. An app that uses the API can fetch originals one by one, and can save the titles, tags, dates and locations that Flickr keeps in its database rather than in the image file.
Flickr has no way to rename a tag. Not on the website, not in Organizr, and not in the API: there is no method that says "call this tag something else". Every rename is really two operations on every photo that carries the tag: add the new one, remove the old one. That is what any tool that offers a rename is doing, and on a tag used by a few thousand photos it is worth knowing before you start.
Flickr's own bulk editing lives in Organizr, the tool linked from the You menu. It can change titles, tags, dates, permissions and licences on a selection, and it works a page of photos at a time. For a few dozen photos it is fine. For a library where the same correction applies to thousands, the limit is not the editing but the selecting, which is what a desktop app changes.
A geotag on Flickr is a latitude and longitude stored with the photo, and it is what puts a photo on your map. Cameras and phones often write one into the file; older scans and many exports have none. Flickr lets you add or fix a location on the website one photo at a time, or in Organizr by dragging onto a map. A desktop app can do it for a whole selection from one point.