If A is devoted to B, A cares about B very much. If A is faithful to B, A doesn't betray B. In a marriage, A doesn't have sex with anyone who is not B. Usually these are similar, but not always. A person might be devoted to his or her spouse, and love that person deeply. But imagine that the spouse is kidnapped, and the person has to have sex with someone in order to find and save the kidnapped spouse . The person is devoted, but unfaithful. OR, imagine that the mob has kidnapped your wife, and will kill her if you betray them. You will be faithful to the mob: you don't want them to kill your wife, so you won't betray them. However, you won't be devoted to the mob.

Devoted- 1. very loving or loyal. e.g. He was a devoted husband. 2. Given over to the display, study, or discussions of. e.g. 'There is a museum devoted to her work' Faithful- 1 Loyal, constant and steadfast. Devoted give the sense of assigning or applying someone or something to an activity, function or end. Faithful implies long-continued and steadfast fidelity to whatever one is bound to by a pledge, duty or obligation.