A website is not a brochure.
...but it can certainly be made to look like one. Most first-timers to the web build their first sites as brochure websites, describing their mission and services, and providing some basic contact information. The resulting site is simple, cheap, and easy to maintain, but it may not be much more than a glorified yellow-pages ad. It certainly does not take full advantage of web technology.
A website is not a magazine.
...but it can certainly be made to look like one. The advantage of online magazines is that they contain informative articles on whatever subjects are appropriate to the site, which helps to attract viewers. The disadvantage is that they may trap you into treating your web pages as static documents that are simply read and then discarded. Web pages are much more than articles wrapped in a slick graphic design.
A website is a communication tool.
Brochures and magazines are communication tools, but they are examples of one-way communication. You push your content out to the viewer, but receive nothing back. More sophisticated websites engage in multi-way communications, which gives not only a richer user experience, but also provides the site owner with useful new data and content. Any time you encounter an online form on the web, you are dealing with a site that receives information from the user, in addition to giving information out. Furthermore, many websites are connected to databases that serve as sources of information to display (such as catalogs, event calendars, and so on). As new information appears in the database, the site automatically grows in scope.
People like free stuff.
An old web adage is that you should offer something for free. The internet is full of free information, and your website is competing against many others for the attention of users. The more barriers you place between the user and the information they would like to view, the more likely they will just go somewhere else to find it. "Free" in this sense means "freedom" just as much as it means "zero cost". For instance, users treat their privacy as having value; if you ask for their e-mail address or name before giving out your information, many users will decline unless they have no other choice. In this age of spam and viruses, one's personal e-mail address has equivalent-to-money value.
If you build it, they won't necessarily come.
Just because you are online doesn't mean visitors will automatically come to your site. Just as you cannot expect to run a booming retail outlet in the middle of nowhere, you also have to ensure that your site is well positioned to draw visitors to it. Website position is not quite as simple in concept as retail location, unfortunately. Good website marketing depends on how you expect people to find out about you. If you are expecting to draw the general public in, then search engine placement may be critical. Getting yourself indexed in appropriate web directories is also important. If you are marketing direct to customers, on the other hand, they need to know your website address, either from your published materials, email, or other means. And of course, once you get them to your website, you also have to keep them there, and possibly even entice them to return again another time.
Keeping content fresh is a big job.
For anything other than the simplest brochure websites, regular content updates are a must. Sites that require frequent or high-volume updates may absorb a lot of manpower keeping the site current. Skilled IT personell (aka. "webmasters") can help, but sometimes this can make the problem worse, since the webmaster can become a bottleneck. Often the webmaster's particular area of expertise (eg. graphic design) won't match the areas of work they get flooded with (eg. editing content, database management). More people for your web team can help, if you're lucky enough to have the budget for that. Otherwise, you should consider how a content management system can help to automate the process of updating your site.
Websites do not have to be built for public use.
Just because websites are publicly accessible on the Internet, doesn't mean they have to be build exclusively for the public. Many websites have private or members-only areas ("extranets") for more private or secure functions. These sites leverage the global reach of the Internet to give a small organization a computer network that can be accessed from anywhere. There are even many hidden sites out there that are built for single-person use; that one person gets their own private computer network tool that can be reached from anywhere in the world at near zero cost -- which is very useful for travellers.
Plan your foundation; don't get distracted by the wallpaper.
Building an interactive website can be a complex undertaking. It is analogous to building a house. You should have a blueprint that includes future forecasts as well as present undertakings. Otherwise you may find that you are not able to add that 2nd storey to your house, and will have to start all over from scratch. When you begin building your site, remember that construction is initially focussed on things like the foundation and layout. The wallpaper and paint (ie. your graphic design) are details that do not concern your construction crew, however much they may concern you as the homeowner. In the view of your contractors, you will be able to change the paint and wallpaper as much as you like once they are done, but if you get the foundation wrong, then your wallpaper plans are irrelevant.
Complex websites are software applications.
Many people view a web page as a simple document, like a page from a magazine (see above). However, if the web page has been composed from information taken from a database or a web form that the user has entered data into, then this document is not so simple. In fact, you also need special software that knows how to take all that information, and assemble it into the web page that the user actually sees. What this means is that a complex website is not really a collection of documents at all, but rather a bunch of computer programs that know how to build web pages on the fly from the content that they are provided with.
If you are managing a project to build your own such website, that means that you are not overseeing the production of documents, but rather the engineering of a software system. It would help to have some understanding of software development processes (especially issues such as developing your business logic and software testing), to better handle the issues that arise.
Pre-packaged web software systems are available that may suit your needs, but their extensibility may be limited. Feature sets may be fixed, or there may only be a specific set of add-ons available. If you need to add new functions, find out if you have to do this through the original vendor, or if you can shop around (or even do it in-house). You should also consider what sort of ongoing support is provided, and what your options are if the vendor goes out of business or stops developing the product.
Interactive websites are like icebergs.
95% of them are invisible. As mentioned above, an interactive website is primarily a software project. Software is all about function, but many newcomers to the web are distracted by websites' form (ie. the visible elements, such as graphics and text on screen). The visible elements of your website may be a very small fraction of everything that is going on there. This also means that your web developers could be making huge strides in your project, with little or no results that are visible to the end user. If you are overseeing a web development project, be sure to understand all these hidden components, so that you can tell the difference between a project that is going like gangbusters on the "back-end", and one that is stalled and in need of help.