Depending on your individual circumstances, you may be able to bring your family or dependents with you during your stay in the UK. However by default you cannot bring your family with you on your visa and need to apply for the ability to do so, you would not be able to bring your extended family however some exemptions are in place in order to bring your partner or spouse with you. Alternatively you can have your family visit you during the holidays on a standard tourist visa which they will have to arrange for themselves.
If you are a parent, you will be able to bring your young children with you (subject to the correct application process), and many universities do offer day care services and nurseries to look after your children during the day while you study. Alternatively you could approach your student support office for advice on childcare facilities and how to find them if the university does not offer such a service.
For students undertaking postgraduate qualifications, there is a separate application process where allowances are made for spouses and older children.
For advice on your particular situation, contact the UK Border Agency for the application that bests suits your needs and circumstances.