One time entry Z visa get turned to Residence permit, and with valid Residence permit you can exit and reenter China till expiration date of the Residence permit.

A work visa ONLY allows you to enter China for the purpose of working for one company once there.  It is a ONE ENTRY visa, and does expires 30 days after you enter the country. 

There are a number of things you MUST DO once you enter China with a Z visa, and PRIOR to start working, and all have time constrains and done at different places. you will need your FEC if not already done, a Health Certificate (takes about 2 weeks to get it), aq work permit and a residence permit. Once you have all of those, you can start working legally even if your WORK VISA expired !.
In regards to the residence permit, it does takes the place of your visa with Immigration Officials. I have seen it both ways, one that does not say anything about entries, and it is consider to be multiple entries, and another that has the slot titled "ENTRIES", and can say 1, 2 or M for the number of entries.
Hope that now you understand how the work visa works in China, and the meaning of the related documents you should have.