In reference to the patent: US7878930
The cited patent is a utility patent. For US patents, the patent number for design patents is prefixed with a "D". Utility patents just have a number (currently in the 9 millions).
Design patents only cover the ornamental design of a functional object, not function. Thus most design patents consist mainly of drawings and very little text. The claims usually just refer to the drawing. Something like "1. The ornamental design for a container, as shown and described.". The original Coke bottle design patent D48,160 is a classic example. This Wikipedia article may be helpful.
Utility patents cover something inventive with utility (duh). There is a need to be novel, useful and non-obvious. Utility patents are usually longer, have an abstract, a background section and detailed description about how to implement the invention. In particular the claims section of utility patents are usually quite detailed and specific in their description of what is patented.