Retirement/Residential Home: these Homes provide accommodation, meals and (hopefully) activities, as well as some support with such activities as bathing and dressing. They are a good fit for those who are still mentally sound but who are becoming frail or unable to feed or look after themselves. They don't generally provide nursing care for medical conditions.
Nursing Home: these Homes provide all the above but have qualified Nurses in attendance at all times. Usually, these Homes are geared towards those with physical needs rather than Dementia, though some will specify that they can deal with Dementia patients. In my experience, these places tended to be quieter, with Residents in their rooms and the common areas empty. The fees tend to be higher (but then the NHS provides a free non-means-tested benefit up to around £100 for nursing care)
Dementia Unit/EMI: those Homes with an EMI (Elderly Mentally Infirm) capability will be secure units with higher staffing levels. This reflects the possibility that the residents are going to try to wander and will need higher levels of support. In my experience, there will be severely demented residents in the common areas, which might be distressing to witness. The assessment you receive from Social Services will tell you if an EMI unit is required.
Physical Disability or Learning Disability Care Home: these Homes are self-evidently for specific care groups, and may well be for other age groups.
Of course, there are some Homes that feature a mixture of the above categories. The place where Mum is currently living is designed as a 'village', comprising a set of common social spaces and facilities surrounded by 3 types of accommodation: self-contained apartments, secure EMI households and a floor of Nursing Care rooms.
If you are looking at Care Homes, make sure you have spoken to your Local Authority who will carry out an assessment of your relative's needs and then provide a list of homes fulfilling these criteria. Also, check the CSCI reports for the homes you find. These will show you what the home is doing right and any areas where they need to improve their service.
My advice is to look at as many homes as you are able. Plenty look great in the brochure or on their website, but you only get a true feel (and smell!) of the place if you visit, and they ought to be able to cope with you visiting unannounced, too.