Does cellulitis require a hospital stay?

Only in severe cases. Small areas of infection can easily be treated as an outpatient or in the clinic. When fever, chills and systemic symptoms occur or the cellulitis covers an extesive area of the extremity or trunk, then hospitalization should be strongly considered.
Depends. It all depends on the severity of the cellulitis and if dead tissue bone etc is visible. For the most part, the majority of cellulitis that i treat is done on an out patient basis.
Perhaps. That depends on the extent & severity of the infection and what other symptoms the patient has (e.g. High fevers, blood pressure issues, dizziness, etc), his/her other medical conditions, and what antibiotics the doctor needs to use. If it is a small area of infection and the patient is otherwise doing well, it can be treated with oral antibiotics at home.
Whats the cause ? Cellulitis is basically a visible inflammation (itis) of the skin and soft tissues (red, hot, swollen and tender) . It can be caused by many things including sunburn (radiation), trauma (sprained ankle) or poor vein circulation (stasis dermatitis). It need not be caused by bacteria. If you have repeated episodes of this, get to a wound care or vascular specialist, the diagnoses you have is wrong.