SEO increases the relevancy of content on the website to the search phrase. SEO helps organic placement in search and is not paid for. To improve SEO, content should be relevant and updated regularly.
Adwords traffic is paid traffic generated by placing money against key words and phrases. Advertisers bid on keywords for their ad to appear when customers search certain terms. SEO also assists paid advertising as the relevance and structure of your website is a factor in how much you pay for your advertisements.
Whilst Google Adwords is effective in getting noticed in search results fast, as soon as the money disappears, so do your ads. Investing in your SEO is a much more effective way to establish long term visibility in search engines.