An architecture is prescriptive, often giving blueprints for solutions, and defining the core building blocks. For instance, "client-server" architecture defines a client, a server, and the roles those two components play.
An architectural style provides a framework for thinking about solutions - in the case of REST, the concept of resources addressable through a URL, a vocabulary for manipulating those resources, statelessness etc.
This is important, because the question "is this a client server architecture" can easily be answered - is there a client, does it connect to the server, are there clear responsibilities for each? The question "is this a RESTful solution" is much harder to answer - it requires you to see if the solution follows the mental model of REST.