One of the fundamental problem with web sites is that they're all too easy to build, but frustratingly hard to maintain.
Creating a web site is about more than just pulling the various items into place and plugging them together. As well as knowing how to put them together, you also have to understand what it is that you're building and why. The same is true when building a house. It is not enough to have the materials on site and knowledge of the relevant construction techniques required to turn them into a building. You must also have a plan to work from which should clearly specify the design of the building. Furthermore if the end result is to be deemed a success then the design must take into account the requirements of the future inhabitants, be sympathetic to its location and conform to any relevant planning laws and building regulations.
And if it's not stretching the metaphor too far, then let's say we'll also need to extend some parts of the building, demolish others and redecorate the whole thing at least twice over the next few years, while simultaneously entertaining thousands of visitors each day...
So that's one reason why I like to talk about web architecture rather than simply web design or development. Many "web designers" are designers first and foremost. They know how to design for the web in addition to the other media that they work in (print, paint, sculpture, etc). Any good designer must have an appreciation for the constrants of the media they choose to work in and the web is no exception. But often the primary focus for a designer is on creating something beautiful with less regard for more mundane matters like how easy it is to use or how simple it will be to adapt, extend or heavens forbid, change the design at a later date.
Others that lay claim to the title of "web designer" aren't really designers at all, but developers who know how to build web sites using their favourite desktop applications, tools or markup languages. They may be highly skilled in the use of their tools of choice, and may additionally have an appreciation for the more technical issues that underpin the web. But if they're not fundamentally inclined towards artistic persuits then they're unlikely to produce a great design no matter how good their tools are or their skill in wielding them.
So without wishing to detract from either group who respectively design and develop some very beautiful and highly functional web sites, allow me to suggest that web architecture is something slightly broader and deeper than these disciplines. It requires an appreciation for the creative element of a web design (how it looks), familiarity with guidelines on usability and best practices in user interface design (how it feels) and an understanding of the underlying technical issues (how it works) involved in realising the design and constructing a web site that looks, feels and smells fresh today, tomorrow and for years to come.
A web architect must find the right balance between form and function to produce a web site that looks nice, is easy to use and is compliant with all the relevant standards. Just as a traditional architect must design a building that is beautiful, functional and doesn't fall down.