Chuck Malcomson from Screwpile communications published an excellent article about the different stages that inbound leads go through:

• Information Qualified Leads - These leads are at the awareness stage. They've shared their "information" in order to get some educational information from you. 
• Marketing Qualified Leads - They've investigated your products or services by reading something related to them. 
• Sales Qualified Leads - They expressed interest in talking to a salesperson about their needs. 

I like the way he's defined these different stages for inbound leads. However, I have one big problem with part of his article. I have a problem with the part where he says that salespeople should wait for marketing to generate sales qualified leads:

"Smart companies will develop a feel for the right time for sales to reach out to their leads. Obviously all SQLs should be called since they have identified themselves as being ready to talk to sales. Calling IQLs may come across as too pushy or aggressive. However, reaching out to MQLs may make sense and help get more leads converted into opportunities that the sales team can work with and bring on board as new customers."

If your salespeople are only interested in talking about their products, then that's the right way to think about it. (You should fire them if that's all they're interested in doing.) Great salespeople are experts and approach prospects in a way that is helpful, regardless of stage. A great salesperson can move a lead from IQL to SQL in 20 minutes. 

The problem is that most salespeople aren't experts at what they do. Assuming they do have the expertise, they also rarely know how to start the conversation in a helpful way. But, it's simple. For example, when a prospect downloads an ebook, a salesperson can call and ask, "I see you downloaded our ebook on xyz. I am an expert at xyz. What were you looking for help with?" This is often the start of a very welcome sales conversation.  A good salesperson should be able to do a little homework about their prospect and find 5 other ways they can be helpful too. 

Don't wait for prospects to realize they have a problem and raise their hand to talk to you because they've already determined your solution is the best. That's not selling. That's customer service. Great salespeople create demand, not just satisfy it. They pick target accounts and they pursue them persistently and creatively. Prospects supposedly conduct 50% of their buying process these days without talking to a sales rep. Let's not make it more than 50% just because we don't know how to be helpful, or we don't know how to avoid being perceived as pushy. 

PS. You might have the luxury of sitting back and waiting for your marketing funnel to deliver sales qualified leads to your inbox. Some agencies actually do and it works for them because they only want to acquire a few new clients per year and their marketing is damn good. But, most companies who are serious about growing, would leave a lot of $ on the table if they sat back and waited for SQLs to arrive.