What is difference between formaldehyde and formalin?

They are essentially the same thing. Formalin is the histologist's term for a solution of formaldehyde stabilized with a small percentage of methanol. In fact any aqueous solution of formaldehyde probably contains trace MeOH. Such solutions would make effective (but very messy) surface disinfectants. Formaldehyde is probably used more often to disinfect equipment in a fumigation-type process, where a closed space is filled with formaldehyde vapor. I think this is done by heat-sublimation of solid para-formaldehyde.