I read a lot about morphs and I sort of understand them now. Now, having said that - I don't know how to use them. How do I know what parts or pmorphs can be used with which figures? I have already made the mistake of getting hair that doesn't work for my characters....

Morph packages are model specific.  You need to purchase them and applied to your base model.  Vendors will usually alert you to which model their characters are based on and you proceed from there.  For example, a V4 character will require V4 morph++ if the mesh is altered to resemble a different feature.

BLF,

If you have hair or other items that do not quite fit your current characters might I also suggest that you read a few tutorials on a poser feature called "magnets".  Magnets are a real godsend, once you practice with them for a bit you can easily make adjustments to things like hair or clothes to fix a lot of issues like poke through, sizing, etc.

It's possible to take hair for one character, like V4, and using magnets resize and reshape it enough so that it fits V3 or other characters.   It's also possilbe to use magnets to create your own morphs for hair, clothing, and pretty much any prop or character.

Bear in mind, you do not need to have your morphs injected into your model in most instances, you just need the morph pack installed and present in your runtime.  Load you model into your scene and apply the INJ morph for the Pose Category of your library.  The installation of morph packagess, such as DAZ's Morph ++ package for either V4/M4, DAZ's Creature Morphs, RDNA's Ultimate V4/M4 Morphs, etc will not visually alter your base models.  They will generate morph dials on your Properties/Parameters palette, by spinning these dials, you will control the degree of your morph as well as combine several morphs. 

Also, installing your basic morph pack will create several types of morph icons in your Pose folder.  You can elect to inject all available morphs into your base model, or you can trim your character's file size weight by selectively injecting only the morphs you are interested in. Afterwards, if you intend to reuse this character again, you may save the morphed character back into the library under the Figures Category (generates a cr2 format file) with a new recognizable name.  You would not need to re-inject your base model again.