Concurrent work is multiple works happening at the same time.
Process is a running program.
Thread (lightweigh process, LWP) is object inside heavyweight process that has it's own control & that shares resources with other threads in the same process.
Critical section is a code section that can be accessed by only one process or thread at the same time.
critical section has uses for example in banking: we do not want data to be overwritten as it's written by other process, or read during writing.
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Source: my partial education (see: ), my experience.
See also: Petersen's Algorithm, Classic Concurrency Problems, Petri Nets.