Nicky Danino (2001) defines HCI as the study, planning, and design of what happens when a user and a computer work together. As its name implies, HCI consists of three parts: the user, the computer itself, and the ways they work together.
When we talk about HCI, we don't necessarily imagine a single user with a desktop computer. By "user", we may mean an individual user, a group of users working together, or maybe even a series of users in an organization, each involved with some part of the job or development. The user is whoever is trying to get the job done using the technology. An appreciation of the way people's sensory systems (sight, hearing, touch) relay information is vital to designing a first-class product.
When we talk about the computer, we're referring to any technology ranging from desktop computers, to large scale computer systems -- even a process control system or an embedded system could be classified as the computer.
There are obvious differences between human interaction and machine interaction. In spite of these, HCI attempts to ensure that they both get on with each other and interact successfully. In order to achieve a usable Website, you need to apply what you know about humans and computers, and consult with likely users throughout the design process. You need to find a reasonable balance between what can be done within the schedule and budget, and what would be ideal for your users.
Human-computer interaction is concerned with the joint performance of tasks by humans and machines; the structure of communication between human and machine; human capabilities to use machines (including the learnabilty of interfaces); algorithms and programming of the interface itself; engineering concerns that arise in designing and building interfaces; the process of specification, design, and implementation of interfaces; and design trade-offs. Human-computer interaction thus has science, engineering, and design aspects.