Most airbag packs use either sealed, single-use cylinder or a refillable, multi-use cylinder (usually compressed air) to store the propellent that helps inflate the air bladders. TSA tightly regulates travel with airbag packs; rules will vary depending on the type of propellant.
There are several basic types of air bladders. Single bags can either be shaped like a 'pillow' behind your head, or with wraparound 'arms' that extend down over your chest for greater protection. Dual-bag systems have a smaller bladder on either side, like 'wings.' This system has the advantage of offering a backup if one bag fails.
The size of your pack will depend on the kind of touring you'll be doing; day-long tours will require a larger pack, while you can go more streamlined for riding heli- or snowcat-accessed terrain. Many bags come with a removable airbag base unit that can be swapped out to a different 'cover' pack according to your need.