Indoor, outdoor, diameter, durometer? All this information can be overwhelming, but there are a few things to know about the differences in wheels that will help you better understand your roller skates (and help you become a better skater).
The main difference between different types of wheels is the hardness as measured by the durometer rating.
Outdoor wheels are softer for absorbing the impact of rough asphalt. Indoor wheels are harder, providing more speed on smooth floors.
In general you can use outdoor wheels on inside surfaces without a problem. You'll skate a bit slower and you'll feel more grip on the floor. New skaters may even prefer this while they're getting the hang of things. In contrast it is more difficult to take indoor wheels outside. You'll feel every rattle in your knees and the wheels will become damaged more quickly.
For your first pair of skates don't overthink the wheels, and when in doubt choose a softer wheel. Many skate sets come with softer wheels because they're more versatile and favored by beginners.
Other Differences
Size (Diameter & Width)
Wheels are available in sizes as large as 65mm in diameter and as small as 32mm in diameter. For the most part, common wheel sizes are compatible with most standard roller skates. You may experience wheel bite (wheel stopping very suddenly) if you use larger wheels (62-65mm) on certain children's size skates or very inexpensive, budget skates. Extremely small wheels (48mm or smaller) are not compatible with most skates, and require specialty parts or modifications to be used.
Larger wheels offer faster speeds and can roll better over uneven surfaces, but they accelerate less quickly. They are preferred for outdoor distance skating, speed skating, and specifically tall, steep, ramps and bowls at the skate park. Smaller wheels accelerate quickly and have better maneuverability. They are preferred for many styles of rhythm and dance skating, and "street" style skate park skating.
Wider wheels provide more stabiliy and grip for speed skating/roller derby. More narrow wheels are favored for artistic and rhythm skating.
If you're just starting out you don't need to think about anything beyond the durometer and size of your wheels. As you decide the specific skating style that most interests you, you may develop preferences for different shapes or wheel formulas. Don't worry about this now, just keep practicing!
