concurrency is structure: many logical tasks in flight, interleaved on however many cpus you have (possibly one). parallelism is hardware: tasks literally executing at the same instant. a single-core machine juggling 400 socket connections is concurrent, not parallel; a gpu multiplying matrices is parallel, barely concurrent. you design concurrency; you buy parallelism (Tanenbaum, Andrew S., 2008).
threads and shared memory
a thread is an independent stream of execution inside one address space: own stack and registers, shared everything else. the sharing is the point — and the disease.