Open chess tournaments are USCF Rated events that usually provide monitory prizes, oppose to scholastic tournaments that provide trophies, medals, certificates as prizes and are specifically intended for K-12 student players.
An Open chess player can usually be of any age and may include scholastic players, unless otherwise specified. An Open tournament might be Rating specific, it could be a closed/invitation-only event.
Prizes for Open events might be Guaranteed at a set amount, and/or based on number of entries, or could simply be percentage-based from the entries paid. At least 50% of entries must be paid back to placing players, and sometimes as much as 90-100% is paid back in prizes.
Due to WKC sponsorship of the Purchase Chess Foundation, most of the Open events provided by the WKC will return 50% of entries.