Adjuvants enhance the immune response, typically allowing for lesser quantities of the viral or bacterial components to be used in the vaccine. Aluminum is an example.
Stabilizers protect the proteins in the vaccine from being degraded during manufacture and transport. Gelatin is an example.
Preservatives keep the vaccine from becoming contaminated. Thimerosal, a mercury-containing preservative, is perhaps the most well-known, although it has been removed from all childhood vaccines except some multi-dose vials of influenza vaccine given to older infants and children.
For in-depth discussions, consult the following Q&As:
“Vaccine Ingredients: What You Should Know” - English [PDF, 817KB]; Spanish [PDF, 832KB]
“Aluminum: What You Should Know” - English [PDF, 296 KB]; Spanish [PDF, 333KB] ; Japanese [PDF, 948KB]
“Is the Aluminum in Vaccines Safe?” – Video
“Thimerosal: What You Should Know” - English [PDF, 193 KB]; Spanish [PDF, 209KB]