What is primer? Think of primer as your base coat to help products adhere better and last longer. As you would prime a wall you are painting for best results, so you prime a face you are painting. It will make a smaller amount of foundation go a lot farther, saving you product and money. Foundation goes on more smoothly for a more flawless finish, and without a doubt, it last more hours and through more environmental effects. For instance, with primer, my blush is still perfectly applied at the end of the day. And, while it doesn't completely eradicate sweat, my makeup stays put better in hot and humid weather.
The primer I talk about when I'm listing these benefits is one of the most popular on the market with makeup artists and plebeians alike: Smashbox Photo Finish, $36. The price is steep, but one bottle lasts me more than a year and saves on foundation costs. With the success of Photo Finish, Smashbox branched off with several more specific primers. They have a hydrating primer, a dark spot correcting primer, a luminizing primer, the list goes on and on so it is definitely a great place to go if you're looking for the right primer for your needs.
It might also be a good place to look when you think about the change in seasons and how it affects your skin. I often apply primer directly to clean skin without a moisturizer, since my skin tends to be oilier. In the winter, I'll often use a hydrating moisturizer to protect against cold weather — give this a think if you're exposed to harsh office heating all winter, too. If you have dry skin and definitely need a moisturizer in the winter and even year-round, you might want to consider a more basic primer, or something mattefying if you feel the moisturizer can leave you shiny.
MAC's Prep + Prime line (around $30 - $40) offers quality primers combined with other cosmetic products like Prep + Prime Highlighter and Vibrancy Eye Primer. These products are great multi-taskers that save you the time and money of using two separate products to achieve the same effect. An eye primer, in particular, is important because not all primers are meant for use on eyes, and it makes a huge difference in helping eye shadow last longer.
The fall and winter season is a great time to try primer since you don't have to worry about the wild card of excessive heat and humidty, and layering products isn't as much of a chore. (Though a good primer like the ones above don't feel heavy.) Consider them for a special occasion and you'll find yourself using them in everyday life.
Via: A Primer on Primer: How to Make Makeup Last Longer