Primarily we mean here a business that partly or completely transacts through a website (either e-commerce or those with a content/advertising revenue model), as opposed to merely using a website to promote your business (ie marketing).
Unless you really know what you are doing, pay someone to build your site for you. Building a website is easy; building a good one is hard. Find a designer who is experienced in your type of site, e.g. e-commerce, community. Then a good brief accounts for at least 50% of the success of a web build project. Many projects fail not because the designer was useless but because the client was unable to articulate what they wanted.
Brief the designer by showing them other sites you like. You might say 'funky' or 'functional', but a designer's idea of what these words mean can be wildly different from yours. Reduce the margin for error by showing them what you want. If your site has big technical requirements, choose a developer who has done a very similar project before. You do not want them learning on your job.
The best sites are interactive. Plan to start small but grow quickly. All-singing, all-dancing on day one always leads to trouble.
In order to stay in the flow, e-retailers must have their product listings constantly updated, attracting users with regular new and relevant content. Content Management Systems (CMSs) are essential if you want to keep your website completely up-to-date. When you are looking for a CMS, don't just accept the first proposal made by any web designer or developer. The market is flooded with CMS solutions offered by a hundred different companies. Many of them are not search engine friendly or do not include elements that should be standard. And if there is one thing you do not want to be behind on your competitors, it is your online visibility.
Search Engine Optimisation (SEO) is the art of creating Web pages that will rank high in search engine returns. Write good content - this is probably the single most important thing you need to do if you want to be found in the search engine. Even if your site is technically perfect for search engine robots, it will not do you any good unless you also fill it with good content.
Sites of different functionality bring different commercial considerations. A site that is primarily a marketing tool for a "real world" business will demand a concentration on branding, naming, design, marketing, PR, communications generally.
An e-commerce site enters into the realms of secure payment processing, customer service, order fulfilment, tracking and delivery etc. A content/revenue generating site will require predicting traffic volumes, dealing with media buyers and networks, advertising revenue forecasts etc.