In-the-Ear Aids  are lightweight devices whose custom-made housings contain all the components; this device fits into the ear canal with no visible wires or tubes. It's possible to control  volume with these aids, so they are helpful only for people with mild hearing loss. Some people find these aids are easier to put on and take off than behind-the-ear aids. Some people find them uncomfortable in hot weather. 

Behind-the-Ear Aids  include a microphone, amplifier and receiver inside a small curved case worn behind the ear; the case is connected to the ear tip by a short plastic tube. The ear tip extends into the ear canal.  They do offer a few advantages. Behind-the-ear aids don't require as much maintenance, are easily interchangeable if they need to be serviced, are more powerful are easier to handle than smaller aids, can provide better sound quality, and tend to be more reliable.