According to the Wikipedia, Information architecture is:
Information architecture (IA) is the structural design of shared information environments; the art and science of organizing and labelling websites, intranets, online communities and software to support usability and findability; and an emerging community of practice focused on bringing principles of design and architecture to the digital landscape. Typically, it involves a model or concept of information that is used and applied to activities which require explicit details of complex information systems. These activities include library systems and database development.
According to the Wikipedia, Interaction design is:
In design, human–computer interaction, and software development, interaction design, often abbreviated IxD, is defined as "the practice of designing interactive digital products, environments, systems, and services." Like many other design fields interaction design also has an interest in form but its main focus is on behavior. What clearly marks interaction design as a design field as opposed to a science or engineering field is that it is synthesis and imagining things as they might be, more so than focusing on how things are.