This describes the different kinds of races in which I most often participate.
A "fixed" series in iRacing is one in which you cannot change the car setup. In real life racing, you can make many adjustments to the car to change how it handles -- for example, you can change the angles of wings on the car, the tire pressures, the stiffness or softness of various parts of the suspension, the angles of the wheels in relation to the car (and therefore the road surface). Fixed series allow you to compete on the basis of driving ability alone, while open series often require finding a good car "setup" to be competitive.
Formula Renault 3.5: This is an open setup series available to B license holders. In real life, before the development of what is now the standard F4-to-F3-to-F2-to-F1 progression, Formula Renault 3.5 was where many drivers competed right before moving up to Formula 1. It is widely speculated that iRacing will retire the Formula Renault 3.5 sometime in 2023 and add a Formula 2 car to take its place on the service, but there has been no indication yet from iRacing that this is in the works.
ARCA Menards Series: A fixed setup stock car series available to D license holders in iRacing. In real life, the ARCA series is 2 levels below the main NASCAR "Cup Series" -- right below the Cup Series is the Xfinity Series (formerly the Nationwide Series, and before that the Busch Series and the Grand National Series), and below that is the ARCA Menards Series. This series races in iRacing on a mix of short tracks (like Martinsville and Lanier) and fast speedways (like Charlotte, Texas, Michigan, and Kentucky). The cars are difficult to drive fast, because their handling is generally very tight (when you turn the steering wheel, the car does not turn) but they are overpowered so that if you push on the gas they suddenly break loose (the back end loses grip and the car "oversteers" or spins).
Formula Renault 2.0: THIS SERIES WAS DISCONTINUED BY iRACING AFTER 2022 S4. This is an open setup series available to D license holders. It is an entry-level series that in real life used to be a feeder series into Formula 1. The cars have a top speed of about 140mph on most tracks. The most difficult aspect of driving them is that the rear wheels easily lock up under braking and the car will immediately spin, usually resulting in a crash.
Indy Pro: This is an open setup series available to C license holders that is the second-lowest rung on the "Road To Indy" ladder that leads to Indy Cars and the Indianapolis 500 in real life. (Above Indy Pro in real life is Indy Lights, which does not currently exist in iRacing, and Indy Cars and the Indianapolis 500, which are available in iRacing.) These cars are more forgiving of overdriving into turns, they'll slide as you brake too hard trying to make the turn, but the slides are pretty easy to catch, unlike the Formula Renault 2.0 car that is difficult to control if you get too hard on the brakes and lock up the rear wheels.
NTT Indy Cars (Oval Series): I drive these a few times each season, but I'm not good at them yet. These cars are so fast and maneuverable and the draft is so strong that everyone darts in and out and around the cars around them, and at the speed of an Indy Car (often 200mph), situations unfold very quickly. Also the "aero push" in these cars makes race driving very different from practice driving.