In Software Architecture - Foundations, Theory and Practice, I can find definitions for both. The problem is that I don't get what each one of them means in plain English:
An Architectural Pattern is a named collection of architectural design decisions that are applicable to a recurring design problem parameterized to account for different software development contexts in which that problem appears.
An Architectural Style is a named collection of architectural design decisions that (1) are applicable in a given development context, (2) constrain architectural design decisions that are specific to a particular system within that context, and (3) elicit beneficial qualities in each resulting system.
What does each one mean and what are the differences between them?
An Architectural Pattern is a way of solving a recurring architectural problem. MVC, for instance, solves the problem of separating the UI from the model. Sensor-Controller-Actuator, is a pattern that will help you with the problem of actuating in face of several input senses.
An Architectural Style, on the other hand, is just a name given to a recurrent architectural design. Contrary to a pattern, it doesn't exist to "solve" a problem.
Pipe&filter doesn't solve any specific problem, it's just a way of organizing your code. Client/server, Main program & subroutine and Abstract Data Types / OO, the same.
Also, a single architecture can contain several architectural styles, and each architectural style can make use of several architectural patterns.