As far as I am aware, there is no legal difference between the terms, but it will depend on their use in the employment contract. Generally, a part time contract will stipulate a number of hours and an hourly rate, adn this number of hours is the *minimum* that your employer can offer to you/pay you for, although dependant on the contract this may work out as an average (eg, 18 hour contract, one week you work 20 hours, another 16 etc). Often you will be required to work over these hours in smaller shops or high street stores, while in supermarkets the hours will regularly be more stable. This does mean you will generally get pay boosts at busy times of year though, such as Christmas. Benefits are often poorer though, with little holiday/pension provision.
Job shares will often be more regular in terms of hours as it is one job split between two people and is less dependant on the company requirements for hours. Job sharers are likely to get the same, or better rights/benefits than part timers. The difficulty with job sharing is often the 'handover' period, as you have to work very closely with one other person and if you dont get on it could get difficult.