An infraction is the breaking of a rule, or doing something that you are not allowed to do. An infringement is also doing something that you are not allowed to do, but specifically means that you are doing something that only someone else is allowed to do. If you build a house badly so that it is unsafe, you have probably committed an infraction of building laws. If you build a house on someone else's land, you are infringing on that person's property rights.