Flyers are small handbills advertising an event or product. They are another great way to spread the word about who you are and what you do.
Flyers are an enhanced version of your business cards in the sense that they can contain not only your contact details but also a little presentation of your brand as well as pictures of your products or descriptions of your services.
You can print them to hand out to people in the streets as a simple front and back (recto/verso → 2 sides), bifold (4 sides) or trifold (6 sides). Some store owners may even allow you to place them on their counters for their customers to pick up.
I will never stress that enough but, flyers — like your other types of communication supports — are a full part of your overall image and will be designed by keeping your visual identity rules in mind.