Categorizing and defining all information/products/documents of an organization is crucial. By drawing out the "rooms" by which a user engages your information is IA.
Nothing is more important than talking to your customer/user directly, finding out their motivations, frustrations, delights... I combine the feedback I get from real users with business goals from stakeholders and a technology roadmap and build my recommendations.
What features does your site offer to the user to interact with your content? Forms, buttons, modals and any interface provided is thought through with detail.
Wireframe - UI Design
Sometimes referred to as Boxes and Arrows, this is the process of getting the ideas into a form that can be viewed by users, stakeholders and your team.