What is difference between cellulitis and mercer infection?

Not much: Cellulitis is caused by multiple different bacteria. Mrsa is the most common bacteria causing cellulitis.

Big difference: 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. Mrsa is simply a subgroup is one of the two common types of bacteria that can cause cellulitis (staph and strep).