Many of your potential customers, funders, volunteers and beneficiaries will find out about your charity by doing a web search.
If you're lucky, that person will already know about your charity and will simply type the name into Google. But, most of the time people will be searching for related keywords, such as 'London cancer charity' or 'fairtrade clothes'. If your website doesn't come up in the first page of search results, then that person will probably go to another website instead.
Although other publicity opportunities, such as print media, are important, having a good search engine ranking is a vital component of your charity's publicity strategy. Not only is the internet always there (while publications have a shelf life), but web surfers are often looking to take action - buy a product, volunteer their time, donate money to a charity - rather than being casual passers by.
For these reasons, you should work towards getting the best search engine ranking possible for your charity's website. This process is known as 'search engine optimisation', or SEO. If you don't have a good search ranking, you may also want to consider using paid search, such as Google Adwords.