A mesh is a 3D representation of a figure. A morph is a representation of changes made to the mesh. The morph doesn't contain all the information of the mesh, just the points that are moved in order to change its shape.
Below is Aiko 3, in her default shape, then with the Realistic morph applied. It's hard to believe it's the same mesh, but it is.
Morphs have values - how strongly the morph affects the figure. Since a morph is simply information about where to move a point, if you set a morph to half strength, it moves all the affected vertices 1/2 the distance. Some morphs only look good at full strength (1.0 or 100% in Poser and DS) while others look good at smaller values as well (.5 / 50% for example). Some morphs can even be used at negative values.
Oftentimes multiple morphs can be used together to customize the character even more, but unfortunately not every morph works well with every other morph. There is no easy way to tell which morphs work well together, it's just a case of experimentation.
Creating a morph can be done in different modelling programs such as Lightwave, Modo, 3DS Max, Maya, Wings 3D, etc. The method depends on which program you use. Poser and DAZ|Studio each have a tool that can create morphs as well. Poser has magnets and D|S has DFormers. Whatever program you use, it must keep the basic mesh information intact if it is to be used successfully when imported into Poser or D|S.
What does this mean?
Let's say for example that you wanted to change Aiko's nose. If the base mesh says that vertices 400 - 600 are the ones that make up her nose, but your modelling program decides to renumber (reorder) the points so that the nose is now vertices 800-1000, when you import and apply the morph to Aiko, it won't affect her nose, it'll affect some other part of her (her left ear, perhaps?) Sometimes the mesh just explodes, implodes, or otherwise looks really painful.
The way to prevent this problem depends on the program.
Another problem that can crop up is when the modelling program changes the number of polygons in the mesh while creating the morph. You CAN NOT add or subtract polygons from a mesh to create a morph. Morphs tell the program to change the location of certain points of the mesh. If you add points or remove them, you no longer have a morph. When you try to import the morph, you may get an error message like "Invalid number of vertices" or the morph might not do anything when applied, or it might actually work but crash the program when you try to render the image. (To me, the last one is exceptionally cruel. "Ha Ha! You thought you got it this time! Neener! Neener!")
The third problem is that more parts are morphed than expected. It can be anything from extra parts of the face morphing to the entire head shifting while the morph is applied. Sometimes this happens because the modeling program picked up extra vertices while the morph was being built. Sometimes this happens because the mesh was somehow changed before being imported into the modeller. If it's the second case, then you can try directly opening up the mesh from the \Geometries\[subfolder(s)] folder instead of exporting it from Poser or D|S.
There are a lot of tutorials out there on how to make morphs and how to bring them into Poser and D|S. I'd recommend starting with Traveler's excellent tutorials and go from there.
