What is the difference between ARM Cortex M7 and Cortex A53, and their possible applications?
The Cortex M series all fit into the microcontroller family, that is they are specifically optimised for smaller, specific, time-sensitive applications. Because they have a more specific job, they typically don't have as much memory.

The Cortex A series are more of a general-purpose embedded type core, that you would find in a smartphone or a tablet or other SOC's. They typically favor higher performance and functions that are more suitable for running an OS such as Android.

Note that both of these are just families of processor cores, they aren't the actual processor bundles including memory and device interfaces. You didn't really provide much information about what you're looking for, but if I had to pick a hardware platform for rich multimedia and streaming, it would be an SOC that had the A-series core in it for performance reasons.