The Z Visa is issued to those who intend to work in China.
Getting a Z Visa in China is particularly tricky, for several reasons. For a start, many companies are not authorized to hire foreigners. This is why, for example, many language schools hire foreign English teachers without providing them with a work visa; lacking the necessary credentials to hire foreign staff, they are unable to get their hands on a Z Visa for said staff without paying a premium (which, of course, they choose not to do, electing instead to illegally supply the teachers with business visas).
Even for companies who can hire foreign staff, the process involves several steps and multiple branches of government bureaucracy. The applicant (or more likely, the applicant's employer) must apply for a work permit ("Alien Employment License") or Foreign Expert's Certificate before the actual visa application can be submitted. This stage of the process involves the employer persuading the relevant authority that the applicant in question has the skills and experience needed to do a job which could not be done equally well by a Chinese person.
Usually, applicants must be at least 25 years old, with relevant educational background - some businesses may demand a bachelor's degree, but this is not demanded by law - and a minimum of two years' work experience in the relevant field, or five if you're teaching something other than languages. Many applications hit a brick wall at this stage.
Applicants must also undergo a medical examination. The official line is that this is to verify that the applicant is not carrying contagious viruses or diseases. However, many suspect that the examination is mainly to test for cases of HIV/AIDS, which is still something of a taboo in China. However, being HIV positive does not necessarily mean that you will be refused a work visa, although it will probably lower your chances considerably.
Applicants may be required to pass a health examination in their own country before a Z Visa is issued. However, many have then found themselves having to undergo another examination upon arrival in China (most cities have a single hospital designated for overseeing examinations of foreigners and issuing the relevant certificates).
When your Physical Examination Record and work permit or Foreign Expert's Certificate have been issued, you (or your prospective employer) can apply for a visa notification letter to be issued. With this taken care of, it's a matter of gathering all these documents, along with those that you are required to provide yourself, and submitting your application to the relevant visa office.
After a Z Visa has been issued, you may now travel to China safe in the knowledge that you will be allowed to enter the country. Upon your arrival in China, the final step in the process for your employer is to take your passport to the local Public Security Bureau (PSB) to apply for your visa to be converted into a Residence Permit (a separate document which will be pasted into your passport).
Up to 90 days for single entry (your Z Visa must be converted into a Residence Permit within this 90-day period). Remember that 'validity' just means how long you have to attempt to enter China on that visa from the day it is supplied, not how long you can actually spend in the country. Your maximum stay per entry is 365 days, and if necessary, you can extend your stay.