IDPA is International Defensive Pistol Association.
It was formed by a some legends of defensive pistol shooting (Bill Wilson, Larry Vickers, Ken Hackathorne, and Walt Rauch) as a counter to the growing gamesmanship in IPSC/USPSA.
They wanted to return practical shooting to its more PRACTICAL purposes. As such, they formed a club that:
1) requires the use of concealed carry style holsters. No skeleton holsters
2) bans the use of compensators
3) requires the guns to use stock controls
Essentially the idea was to lower the cost of entry and increase the applicability of the skills developed by playing the game to actual defensive pistolcraft.
In short, its the other large and influential practical pistol league.
SSP or Stock Service Pistol is the most popular IDPA class. Its the class that most people who shoot Glocks, S&W M&Ps or any other traditional double action or double action only pistol in 9mm or .40 falls.
The classification is based on a standard course of fire. The order goes
unclassified
marksman
sharpshooter
expert
master
grandmaster
So a classification of expert shows that you have some fundamental skills, but are by no means exceptional.
Don