Paraformaldehyde (PFA) is a polymer of formaldehyde. Paraformaldehyde itself is not a fixing agent, and needs to be broken down into its basic building block, formaldehyde. This can be done by heating or basic conditions until it becomes solubilized. 

Formalin is the name for saturated (37%) formaldehyde solution. Beware though, some commercial formaldehyde solutions contain methanol to prevent polymerization (into paraformaldehyde). Since 100% formalin contains up to 15% of methanol as a stabilizer, it has a significant impact on cell fixation.