​WHAT IS ORTHOSIS?
An orthosis (or orthotic) is an apparatus that supports, aligns, prevents, protects or corrects deformities, or improves function for a section of the body. Most orthoses, like a back or knee brace, "wrap” or cradle the body area they're intended to help. A foot orthosis, however, is unique. While its goal – improvement – is what every orthosis aims for, a foot or a foot-and-ankle orthosis can't be "worn” like other orthoses. A foot or foot-and-ankle orthosis can only achieve its goal when working in conjunction with the appropriate shoe.
Pedorthic Footcare Association (PFA)

WHAT ARE CUSTOM ARCH SUPPORTS?
Many problems in the feet respond to stretching and conditioning, choosing a different shoe, and simple over-the-counter shoe modifications. However, long-term (chronic) and complicated problems of the feet may require specially designed inserts (orthoses) made of materials that concentrate relief on a particular area while supporting other areas. Severe flat foot, high arches, shin splints, Achilles tendinitis and turf toe are but a few of these conditions. To obtain the best relief for such problems, see an orthopaedic surgeon, a doctor specializing in diseases of the bones and joints. The orthopaedic surgeon is trained to treat problems of the foot and ankle. Pedorthists and orthotists are trained to make and modify arch supports (orthoses) and fulfill the surgeon's prescription. Working with these professionals will ensure you get the right shoe for the best possible treatment.
American Orthopaedic Foot & Ankle Society