A chicken sandwich consists of chicken, either a fillet or patty, various toppings, and bread.
The meat can be white or dark meat (usually thigh meat is used when it's dark meat) and be shredded, deep-fried, roasted, or grilled; even cold cuts can be used. On the other hand, bread can be any bread, either bread slices cut from a loaf, a bun, or a roll.
There are many types of chicken sandwiches in the US, such as Ohio's shredded chicken sandwich. Also, hamburger joints often offer chicken sandwiches as an alternative to beef. Those sandwiches are prepared the same as hamburgers and can be found on menus under such names as "chicken burger" and "chicken on the bun".
But when it comes to famous variations worldwide, there are also many. For example, in Ireland, there's the chicken fillet roll consisting of a baguette filled with breaded and fried chicken. In Eastern Canada, there's the hot chicken sandwich which is a chicken sandwich covered in gravy, often accompanied by peas, and eaten with a knife and fork.