To have a successful Google Adwords campaign, its important to identify keywords that are particularly relevant to your business as well as being cost effective. The Google Adwords Keyword Tool can help identify keywords that are related to your main business.
You can either enter a keyword phrase to find related keywords, or you can enter your website URL (or your landing page) and Google will then identify useful keywords! Below you can see an example of the usage of this tool. I chose to enter my website URL (green oval) as well as including other pages on my site. Google then correctly identified different relevant categories (red box). The first few results from the first category, Pay Per Click, are shown. The completely green Advertiser Competition boxes indicate that these keyword phrases are highly competitive and thus expensive. The Search Volume columns indicate the traffic on a monthly basis.
Even from an SEO perspective, this Keyword Tool is extremely useful, as it shows you what Google thinks your website is all about.