This is a very broad category and, in reality, a subcategory of spray applied products. The operative word in this is, protection. There will be a future article discussing spray applied paint cleaners, but for the sake of focus, I've subdivided the category at a logical point.
It should be noted that many products cross boundaries between cleaner and protectant. Even some discussed below can be used in a pseudo-cleaning manner. I've futher broken the category into several groups based on the formulation.
Spray Applied Waxes - Though many products will call themselves a spray wax, it is extremely difficult to get any appreciable amount of wax into a spray-able form. Such products require a micro-emulsion of carnauba or other wax for this to be achieved. In general the wax will be accompanied by synthetic polymers that are more often found in sealant. It should be noted that even in this case, higher viscosity polymers will not spray well in marge quantities by their very nature. As such, a spray wax is limited in it's potential formulation. These products are characterized by a slight darkening effect and generally have poor durability. It should be noted that a higher polymer content, coupled with reactive silicones, can contribute to longer durability. But, with no way to actively bind the wax molecule, the look may degrade while the performance remains. There has been some development in polymer waxes, that is a wax that has been bonded to a polymer capable of cross-linking, though these are rare.
Spray Applied Sealants - Similar to liquid sealants, these products are purely or predominantly synthetic. They are also often based on polydimethylsilozane (PDMS) and can contain reactive silicones just as a traditional sealant. As stated above, the polymer make-up is limited in viscosity due to the nature of the product as spray-able. Similar to traditional sealants, they will typically be characterized by a sterile high shine and generally have better durability than they wax counterparts. Some of these products are true sealants and can offer months of durability for minimal effort, others may only last a week or two. A sub-section within spray sealants are wipe-on walk-away sealant. As the name suggests, these products simply need to be wiped on, rather than allowed to haze and be buffed off, saving additional time.
Silica Based Spray Sealants - These products take a step into the coating realm, utilizing similar SiO2 molecules, though generally without the carrier resins that aid in the durability of a true coating. They are charactarized by a glassy look and extreme hydrophobic behavior along with months of durability. They vary from easy to apply to challenging and there have been cases of bad batches from certain companies that result in a very streaky and poor finish that is difficult to resolve. These products can be used as stand-alone protection, but are also used as toppers for coatings to avoid water spotting and re-establish hydrophobic performance.
Touchless Spray Sealants - Though detailers often think of these are being strictly silica based, they were first introduced in tunnel and touchless car washes. These products are generally sprayed on and rinsed off. This is a good options for those looking to save time or avoid touch their paint. Products available at tunnel car washes tend to have very short durability. Those available from detailing outlets often contain silica and can last for several months. In my own testing, I've found some can last through several application of wheel acid, where a normal paste wax or liquid sealant would fail. In this respect, these can offer very robust and effective protection.