Just to share my general view (
without taking into account the differences of dial, movements, special edition, limited edition, boutique edition and special functions like chronographs etc):-
Luminor / Luminor Marina - These models are more rugged and sporty. It has the signature Crown Guard of Officine Panerai which is a Registered Trade Mark. The word "Marina" will normally means that it has a seconds hand. Just to name a few famous examples which I have learned during the course of this year:- PAM 26K, 000, 005, 111, 112, 127 Fiddy, 195, 203 (the holy grail with 8 days movement), 217 (famous MM dial), 312, 320, 351, 360 (the community watch for Paneristi), 372 (the most wanted watch in year 2011-2012), 390 (the very nice boutique edition), latest release like PAM 422 etc.
Radiomir - These models are more dressy and elegant. With the exception of PAM 341 Egiziano which has the crown guard, these Radiomir models are normally with screw in crown only. The more simple it is the more beautiful. Examples are: PAM190 (famous with JLC 8 days movement), PAM210, the ever stunning PAM232, PAM292, PAM380, PAM339 (who can forget this beautiful browny!), new SIHH 2012 release like PAM399, PAM424, PAM448 (California Dial), PAM449 (SLC) etc.
Submersible - These Models are the real divers version of Panerai. It also has the signature Crown Guard of Officine Panerai. The water resistance are generally high. They look huge and rugged. Some made of titatium to reduce the weight. Panerai has just released a Titatium and Ceramic version of this Submersible watch namely PAM389, very beautiful indeed. Few examples are: PAM24, PAM305, PAM 371 and PAM382 (the famous Bronze watch).
Mare Nostrum - This is one of the most distinctive looks of any Panerai. As kindly shown and posted by MnazriTan in his post. These models are much more classic and regarded as more "vintage". History goes back to 1940 and the watch was provided by Rolex for the use of Italian Navy. Examples are: 5218-301/A, PAM 300 etc.
Ferrari Edition - These watches are by its name self explanatory made for Ferrari. They don't look like Panerai at all. Generally very expensive. No examples of my head as I am less interested in these models. The references normally like FER001 FER002 etc.
Purdey / Sealand for Purdey -These watches have a lid to cover the dial of the watch. Created for James Purdey & Sons. The lid has engraving of like lion, tiger etc. No examples of my head as I am less interested in these models.
With the above categories, you can choose what you want and start from there.
Say for example you want a diver's watch, then go for Submersible.
If you want a dressy watch then go for Radiomir.
If you want sporty and manly look then go for Luminor.
Mare Nostrum is for real collector, I believe at least.
Ferrari and Purdey not my cup of tea.
There can some special edition like "Daylight" and "Slytech" etc. These are just like the "Legacy" and the "Rubben Barrichello II (RBII)" in Audemars Piguet. Nothing too difficult to understand really.
Having said the above, this post consists of my general view only, the list is not exhaustive. I do not own a PAM just yet. I have started to learn above Panerai only this year and I am still learning and find it interesting to learn in many aspect.
For those who are interested, there is a website with namely Paneraisource.com which is very helpful if you want to check out the list of models of Panerai. In fact I started to learn from this website and slowly go into public forum. (Believe it or not, I have clicked on all the watches on the list just to check it out!)
I spent my lunch time just writing this post! Hope this assists! Haha...