It is hard to define precisely, but the majority of jobs are fairly obviously either full-time or part-time. Generally, jobs with fixed core working hours, over 30 hours per week, are full time.
Jobs with variable working hours and/or less than 30 hours per week are usually considered part time. In theory somebody can do a part time job leaving time for them to study (full- or part time) or look after their family. Obviously there will be people for whom part time work is neither convenient nor enough.
Some jobs, for example in catering, farm labour or the building trade are seasonal and/or very variable - sometimes from one day to the next. These may be full time or part time; workers are generally committed either working or waiting for work. Good luck could mean comstant employment, bad luck to boredom or starvation, classification into full or part time is potentially arbitrary.
There is no direct relationship between shift working and part time working; many full time jobs (policing, nursing, transport, manufacturing) involve shift work, but for individuals who have a full time 'day job', they may be able to fit in a part time job ay night, their part time work is therefore shift work.