With such questions as these you are probably best talking to your VAR so that you get a proper understanding of what is involved. Other than that, perhaps read the SolidWorks reference material on PDM Standard.......
These are not questions that have simple answers to give you a proper understanding.
There are two types of PDM in SW. One is workgroup PDM & other is Enterprise PDM. Workgroup PDM is suitable for small setup of around 7-10 computer and EPDM is suitable for large setups i.e., more than 10.
You can actually define a total life cycle of product in PDM. From starting of raw material to all the mfg process. Its just kind of vault where you check in and check out documents like parts, assemblies, drawings. But let me tell you it is very very useful tool. One can actually keep track of part that what changes have been made by which user on which date and who will be the authorized person for approve these changes, when the person has approved it, did the changes are fine or need more work on it etc.
And you can always set parameter by going start>>PDM. There are lots of paramters you can use. Mainly there are two types of users. One is admin and rest is other. Admin has all the rights which others don't have.
All in one its very very useful tool. Just go for it. Let me tell you that we are using WPDM for only 2 users. Just because we find it very useful.