Hoh boy--you got a spare year? It's gonna take that long just to explain the different types of plugin on the market, far less explain how and where you would use them.
In a lot of cases, plugins exist only to make the user's workflow, whether they be restoring old photographs, archiving shots they've taken themselves or collating commercial photographs for use in new artwork, easier. Most (if not all) effects obtainable with plugins can be obtained with straight Phoptoshop, *IF* you're prepared to put the work in (and obviously, given that you know the app intimately). So, working from that given standpoint, you then have to decide just *WHAT* you want to do with the plugins.
There's no magic bullet here--you're gonna have to do a LOT of research, experimentation and messing around till you get exactly what you're after. That said, by browsing the manufacturer's websites (Alienskin, OnOne, Flaming Pear, AutoFX to name a few) you can easily find out just what specific plugins are designed to do. The trick then is to figure out whether that function works into your own workflow.
I personally do a lot of photo restoration and straight graphic work, so I can normally find uses for darn near any plugin out there. Others have more restricted needs/wants, and they then need to take what's available out there as a given and figure out how/where/when those plugins could be of use to them.
If you're anything like me, you'll start browsing the plugin manuf's websites, and ideas will start popping into your skull as you check out what the individual packages can do for you. Trust me, you'll find uses for most of the commercially available suites out there--after all, that's what they have been designed for--to appeal to the widest possible audience.
In other words, you're gonna need a couple of things that most folks nowadays find in short supply--time and money. Time to find out exactly what your needs are and how you're gonna address them, and the cash to invest in the software once you've made that informed choice regarding what you're going for.
Feel free to contact me offlist and I'll offer what adviuce I can. However, let me add a rider to that. Obviously, there's no way I can know up front what you're trying to do, or how your workflow is going to affect your choice. Only YOU can make that determination.
Hang on to your hat,--you're in for quite a ride!!!
Hope This of Help
Scots