Hi April,
Social bookmarking is a way for people to bookmark sites they visit and enjoy, often employing the use of tags for easier organization, that they can share with other people.
Some social bookmarking sites include
Delicious,
BlinkList,
Magnolia,
Stumble Upon (not fully a bookmarking site in the traditional sense, but it allows people to save and share various websites/resources they find), and others.
Each site has their own system, but usually the sites that bookmarked by the most people will move higher up in the rankings (most notorious is the Delicious Popular List). But, not all sites use this system.
Social bookmarking groups are springing up as a way to help each other boost websites/pages on their sites. Some people see it as unethical if they simply bookmark spam pages and sites, otherwise if it's just a group of like-minded people sharing helpful information, then go for it.
Since I don't know which Yahoo! Group you're referring to, I can't provide any additional info on it. But, nonetheless, I hope what I did add has helpful.
~ Teli