Cosmetic products are known to have a short shelf life if nothing is done to extend it. This is where preservatives and antioxidants come to the fore. Oftentimes they are used interchangeably by those who think they do the same thing, but they do not and cannot be substituted for each other.
When developing your formulation you may have a water phase and/or an oil phase, both of which have their own issues which need addressing.
The water phase will introduce the very real possibility of bacteria, mould and fungus developing in your product. This is the type of problem we use a preservative to combat. When added to the product (usually in the cool down phase or when cold) preservatives stop microorganisms from being able to effectively grow in your water based creations. These can include creams, lotions, micellar water, emulsified scrubs, toners etc. Keep in mind that spores are present in our very air and on our skin, so contamination is near impossible to avoid. Adding preservatives such as Optiphen Plus, Liquid Germal Plus and Euxyl PE9010 will allow your product to stay germ free for a considerable length of time when used at the recommended rate.
The oils you use in your products are a different situation. Oils are not prone to aerobic bacteria, so do not need a preservative like those containing water. They are, however, prone to oxidisation caused by exposure to the air. To help reduce the "aging" of your oils an antioxidant is recommended, such as Vitamin E or Rosemary CO2 Extract. When used at the suggested rate oxidation of the oils will slow to extend the life of the products made with them. Be sure to add when the product is below 40ºC for best results.