IS Sets (short for Invitational Series) are 14-packet (formerly 15-packet) question sets produced by NAQT, typically for use at regular season high school quizbowl tournaments. Each packet consists of 24 tossups and 24 bonuses; the distribution is posted on naqt.com. Generally, each IS set is used by many different tournaments around the country on different dates during the school year. As of the 2010-11 season, NAQT produces six new IS-sets per year.
Introductory Invitational Series
NAQT also produces 12-packet Introductory IS sets, also known as "A-series" sets or A-Sets, because their designations end with the letter A (such as "IS #97A). These sets have shorter, easier questions than the regular Invitational Series and are intended for first- and second-year high school players. NAQT produces four new A-sets each year.
Until about 2009, some elite high school teams criticized IS Sets for being too easy, or for being funn due to a high amount of cutesy or non-academic writing. Though the quality of the writing has improved recently in tandem with good quizbowl standards, statistics-based inquiry has shown that IS-sets since then are generally more difficult for local/low-level teams than other available options (see here). Despite this, IS-sets are the staple of many local high school quizbowl circuits, with overall reception being positive in recent years.
College tournament controversy
Many people prominent in the college game have criticized college tournaments for using IS sets. Their arguments include the fact that questions produced for high school players are inappropriate for a college audience, and that these tournaments might sap funds from tournaments aimed at college teams. Though many tournaments such as CUT and WashU's annual GIT once used IS-sets for college teams, the practice has declined over time. See these threads for discussion of this issue.