For the most part content is the words and images that you use to convey information about your company or organisation and its products and/or services. Effective, useful and unique content is key to the success of your website.
Having good content is the key to getting good results in search engines.
For a business, detailed and interesting descriptions of your products and services assist your customers in making a purchasing decision. Having additional content that is related to your product or industry also helps, because if the content is good quality it will increase your customers' opinion of you.
The more useful and interesting the content is, the more likely it is that people will want to read it, and the more often you update your content, the more often they'll want to come back to see what's new. Having unique content (or combining public domain content in a useful and original way) instead of merely copying or buying content from another site gives your visitors a reason to come to your site instead.
Writing for the web is different to writing for print. Thoughtful use of headings and subheadings are important, because people reading on the web often scan text rather than reading it in full. Web usability guru Jakob Nielsen says that 79% of users scan text rather than reading it word for word.
Headings should summarise the main point of the text that follows them. This lets people skip information that they're not interested in and makes it easier to find information that they are interested in. Keep paragraphs as short as you can while still getting your message across clearly.
Try not to make your content too dry. It's easier to read something written in an informal, active tone than it is to read something dry and formal. Try to avoid jargon and technical language except for when you absolutely have to. Remember that you're trying to communicate information, not impress people with your vocabulary.
Avoid using marketese (language that makes exaggerated, subjective claims). People don't like it, they'll see through it, and it makes you seem less credible. Earn their trust by giving them information that they want to know rather than telling them what you want them to think.
It doesn't matter how experienced a writer you are, if you can provide the raw information, a copywriter can always flesh it out, with further research if necessary. This is a great solution for people who don't have the time or desire to write content themselves.