Vendors usually mix several tools to fill a season, and each one answers a different question. Knowing which tool answers which question keeps you from paying for overlap or missing a show that only one of them lists.
A directory or calendar answers the question of which shows exist, and when and where they are. The useful ones list dates, location, booth fees, application deadlines and a link to the organizer, and let you search by state, city, month or type. Find A Market is one of these, built around searching by place and date with fees and deadlines on the page.
Directories differ in how they collect listings, how often they check them and how much they show without an account, so it helps to compare two or three for the area you sell in and note where they disagree.
An application platform is where you actually apply: you upload images, answer the jury questions, and pay the application fee in one place. These are most common for juried art fairs and larger festivals. They do not replace a calendar, since they only show the shows that use them, but they save you from re-entering your details for every application.
Regional vendor groups on social media answer the question of what a show is really like: how the load-in went, whether attendance matched the promotion, and whether the organizer pays attention. They are the best source of recent firsthand reports and last-minute openings, and the hardest to search by date or fee.
Save listings, keep application deadlines in one place, and read what vendors who have worked a show say about it before you commit to a booth fee in your area.