Web architecture is a distillation of several academic fields:

• web usability,
• information architecture,
• user experience design,
• interaction design,
• user interface design,
• human-computer interaction,
• information design,
• web design, and
• graphic design

(Academics like to make a lot of distinctions.) To keep things simple:

Web architecture means structuring a website to achieve business goals.

This involves a mix of:

• Business coaching to set goals for the website
• Keyword and competition research, both for SEO and content development
• Content development to gather the raw information necessary to support goals and to deliver customer value
• Architecture to create a website based on business goals and behavioral psychology
• Website layout and design
• Website construction and development
• Search engine marketing, e-mail marketing, and social networking to build an audience

Web Designers are not Web Architects

Most web designers don't do any architecture work. They leave the architectural decisions to the client and just build what they're told to build. A designer is the equivalent of an interior decorator. Would you trust your interior decorator to draw up the blueprints for your dream house? Would you trust yourself? That's what an architect is for.