There are two basic types of architecture: Von Neumann & Harvard. In most of the Microcontrollers, often Harvard or modified-Harvard is used.
Von Neumann: Von Neumann architecture has a single, common memory space where both program instructions and data are stored. There is a single data bus which fetches both instructions and data. Each time the CPU fetches a program instruction it may have to perform one or more read/write operations to data memory space. It must wait until these subsequent operations are complete before it can fetch and decode the next program instruction. The advantage to this architecture lies in its simplicity and economy.
Harvard: Harvard architecture has separate memory areas for program instructions and data. There are two or more internal data buses which allow simultaneous access to both instructions and data. The CPU fetches instructions on the program memory bus. If the fetched instruction requires an operation on data memory, the CPU can fetch the next program instruction while it uses the data bus for its data operation. This speeds up execution time at the cost of more hardware complexity.