Your iRating is a measure of your skill as compared to other iRacing.com drivers. iRatings are used to ensure similarly skilled drivers compete against each other in official sessions and Race Series. In a nutshell, based on your iRating when you enter the race, and the strength of field for that race, if you finish below where we feel you should finish, your iRating will decrease and if you finish above where we feel you should finish, than your iRating will increase.
All and only official races (TTs, Qual, Practices have no affect on iRating) will affect your iRating in their respective category, oval or road course. Hosted races and some Special events like the 24 Hours of Fun do not affect the Safety Rating or iRating.
Your iRating is category specific. Only Official Race sessions in each racing category affect your iRating for that category.
Your iRating and ttRating will only appear on your competition license once you are awarded your Class "D" license.
You may find some additional information in section 4.1 of the Sporting Code.