Self inflating sleeping pads combine open-cell foam insulation with a valve that can inflate or deflate automatically. They're often ideal for camping where you'll be sleeping in a tent or camper and want an easy setup.
Some pads can inflate and deflate on their own with air from your breath (though some may require a pump). Others use a twisting plastic valve that requires you to turn one direction for inflation and the other to close it, which can be hard on tired hands or tongues.
You'll typically find self-inflating sleeping pads ranging from 1.5 inches to 3 inches thick, depending on how much comfort and insulation you need for your sleeping style and the terrain you'll be camping in. Thicker self-inflating pads offer more comfort, but they're heavier and bulkier than thinner options.
Weight is always a key consideration when choosing a sleeping pad. The heavier a pad is, the more it will weigh in your pack. A lightweight pad can save you a lot of weight and space when you're backpacking or car camping, but it can also be more uncomfortable than a heavier pad.
Inflatables are usually easier to inflate than self-inflating pads, but they take more time to do so. They can be a bit crinkly and noisy when inflated, which can be an issue for people in the same tent.
Lastly, inflatables can be prone to punctures, so carry something to patch your pad with it.