The above question should be answered in first 3-6 seconds , else statistics shows that the users leave the site.
Explain in short / brief what your site is about in 5-10 words. Preferable a slogan under your logo or adjacent to your logo. Some sites are explainatory by domain name eg, however domain names not always give visitors idea about site purpose.
To summarize you can use following methods to convey your site purpose to visitors.
2. Mission statement
3. Flash introduction video
4. AJAX based windows which open on the home page only for first visit.
5. Embedding a video on the first page.
6. Embedding a audio on first page.