Inbound Marketing is a method of marketing that focuses on getting found by prospective customers.
In a sense this is related to relationship marketing and Seth Godin's idea of permission marketing. David Meerman Scott recommends that marketers "earn their way in" in contrast to outbound marketing where they use to have to "buy, beg, or bug their way in. Other terminology is Attraction marketing and Next Best Action Marketing.
Inbound marketing is useful to fill your Sales Pipeline with more Qualified Leads and thus you can obtain greater Success with CRM by the use of an Inbound Marketing strategy.
Inbound marketing involves the use of social media marketing and inviting conservations to happen - when the customer is ready.
Outbound Marketing:
Outbound (traditional) marketing is focused on interrupting the prospective customer at some planned time or schedule. It involves direct mail, TV, Radio, print of any kind or more traditional approaches to marketing. In fact, a static traditional web site is like a print brochure on the Internet.... but at least it may be found on the some Google page.
Traditional outbound marketing techniques are becoming less effective. Buyers are not only finding ways to tune these messages out, but more importantly they now have the capability to evaluate the products and services they need on their own.
As a result, businesses are transforming their marketing efforts to focus more on inbound programs that allow customers to find them. (ie...social media marketing)

Elephants at WaterHole are like Customers searching on the Internet:
A great analogy is that traditional marketers look to garner interest from new potential customers they act like lions hunting in the jungle for elephants. The elephants used to be in the jungle inthe 80's and 90's when they were learning their trade, but they dont' seem to be there anymore. They have all migrated to the watering holes on the savannah (the internet). So, rather than continuing to hunt in the jungle, I would recommend setting up shop at the watering hold or turning your web site into its own watering hole.

Two Key Questions:
Where are your prospective customer to be found on the Internet?
When they need to find information and make a decision to buy can they find you or do they find your competitor?