A deep, thoughtful analysis of competitive advantages, walking through and differentiating each type:

An innovation is a type of competitive advantage (though not all competitive advantages are innovations) and the strategic job is to make that competitive advantage sustainable over time1, to maintain the advantage as a bulwark against competition. Mechanisms to deter or slow imitation are called, colloquially, moats. Moats draw their power to prevent imitation from one of four basic sources:

• The state,
• Special know-how,
• Scale, or
• System rigidity.

Bookmarking this one for future reference. An excellent baseline.