Croutons are small, roughly cubical pieces of toasted bread, often with some seasoning. Dry toast is toasted bread without butter or any other spread.
According to Wikipedia, a rusk is "...a hard, dry biscuit or a twice-baked bread." We always called this Zweiback in my family when I was growing up. Toasted bread is bread which has been dried and browned, usually in a toaster.