I keep hearing that constantly adding fresh content to your site improves your ranking because somehow Google/Yahoo is checking this. How exactly would they be able to check that?
The explanation depends on how often you add fresh content to your site, if you make constant updates the spiders will visit your site often. You can find more info here
I think that fresh content is related mostly to text and images, so banner is a way of fresh content but few paragraphs of text would do better.
Have you heard about crawlers? Yahoo Crawler Google Bot, these are called search engine spiders. They daily visit Billions Trillions of websites from all over the world and for Blogs ping services inform them about new content. If you have installed some tracking tool you will see everyday search engine spiders crawls your website.
If you have a blog connected to your website like www.blog.yoursite.com and you submit new blogs to it would that be considered new content?
if you have fresh content when the spider visits, they will speed up the frequency of crawls - if you don't, the frequency will slow down again.
Yes, Adding up of fresh content to your site will make the spider keep on visiting the site and with that the chances of making the crawler stay on the website is good.
Its well proved that google like new and unique content and often crawls with it.The best example i can give u is of my website - from last five days i aws writing contents and submiiting to directories and you know what i got two baclinks,although they were of yahoo';s.
Google like fresh content and adding fresh content to your website give spider a reason to visit your website often
once you post a new content, i suggest you ping it...helps to get new content index and crawl easily...
Google likes unique content, if you make the fresh content then spiders are cwral your pages, it will helps to improve your site rankings.