Ben... Selling a tangible product usually requires a demonstration or presentation which proves or shows the benefit of ownership of the item. This is mostly performed after you've discovered the initial interest of the prospective customer in the product. Examples could include: computer, car, house, clothing, etc. Most of these prospective customers come to you and already have a need for your product.
Selling a service, on the other hand, requires more of that "trust factor" between you and the prospective customer because they are relying on YOUR words and mental pictures which you create in their mind of the final benefit they will receive from the commitment to purchase. In this case you are using stories and testimonials of past results rather than demonstration.
Most of these prospective customers have to be sought out by you and do not fully understand that they need your service. Examples of this type would be insurance, marketing services, preventative health care, construction labor intensive services, investment management, etc.
Selling the service or the "intangible" is more difficult and requires more sales ability and skill, therefore it usually commands higher commissions or rewards.
The best paid sales professions are usually a mix of selling both. An example would be technology systems that need analysis, hardware, software, design, configuration, installation, service, and support.