Sleeping pad is the most important sleeping equipment right after sleeping bag. The main purpose of sleeping pad is to isolate the sleeper from the cold ground. The most essential features are insulation, packability (size and weight), durability and comfort, between which you need to find the balance.
When you are sleeping in a sleeping bag, the weight of your body flattens the filling of the sleeping bag, preventing its special features. You often meet the misconception that a warm sleeping bag is enough. In cold winter weather, a sleeping pad with good insulation is essential and helps you sleep better. Insulating sleeping pad is necessary in cold weather because it reduces flattening of the filling and creates an insulating layer between sleeping bag and sleeping pad.
Sleeping pad's resistance to conductive heat flow is measured or rated in terms of its thermal resistance or R-value. The higher the R-value, the greater the insulating effectiveness.
When sleeping in summer, the R-value doesn't really matter.
R-value of sleeping pad used in autumn and spring (3-season) should be at least 3.
R-value of pad used in winter (4-season) should be at least 5.
Solid foam pad is made of cellular plastic or rubber, and it is light. The thickness varies from 0.5 to 1.5 cm. The advantages of the foam pad are reliability and durability. You don't need to fill it, but it is ready to use right away. On the other hand, it is not very soft or comfortable. It takes more space when packed than air-filled pads, so it is usually tied outside the rucksack. Foam pad is usually packed by rolling, and it can have different surfaces to improve insulation.
The advantage of air-filled pad is the fact that it is comfortable and it can be packed in a smaller space. On the other hand, air-filled pad is heavier and not so durable. The lightest air-filled pads weight only 300-400 grams. Air filled pads often have extra filling to improve the R-value. The extra filling can be synthetic fibre or down.
Self-inflating pads have open cell foam that expands when the valve is opened. You often need some kind of blow too before the pad is filled completely. Inflatable pads are filled entirely by blowing or with a pump.