On most social media sites the pound sign '#' turns any words that directly follow it into a searchable link. Hashtags allow you to group your content into topics based on keywords.
When social media is done effectively its importance cannot be argued. In order to use social media effectively you need to be able to focus time and energy on it. Just like every one of your clients requires a different approach, each social media outlet should be used differently and attracts a different type of audience.