Snapchat originally designed to be used for private, one-on-one photo sharing, but it can also be used for a variety of tasks, including sending films, live video conferencing, messaging, creating caricature-like Bitmoji characters, and broadcasting a chronological 'story' to all of your followers. There's even a 'Discovery' section where you can find short-form content from prominent publishers like Buzzfeed. Snapchat even allows you to store media in a secure location.
Other capabilities include the ability to apply filters and AR-based lenses to photos, as well as the option to view your current location on a world map. However, the most important thing to remember about Snapchat is that it is all about quick communication via your phone.
Before Snapchat, social networking was mostly a desktop-based experience. For example, you could publish status updates, tweets, and images online, and your friends could remark on them, and you'd all be able to see them forever.