Architectural patterns are meant for providing solutions to architectural problems in software engineering. It fundamentally provides a readymade structure of a solution that is targeted at a particular established proble. It describes a structural organization schema for a solution consisting of subsystems and underlines their responsibilities and interrelations.
However there is significant difference between the architectural pattern and the architecture itself. An architectural pattern is a concept that captures essential elements of a software architecture. The pattern is a guideline to implement a particular architecture. So following a particular architectural pattern, the architects may come out with several different and unique architectures. Of course they will carry some similar set of characteristics.
One way of depicting the same is that Architectural pattern is an abstraction (interface in .net?) and actual architectures are implementations of the same.
So how can we compare architectural patterns to design patterns? Architectural patterns are larger in scale and gives an overall structure of the solution. There might be several design patterns in a single architectural pattern.