Just as the class P can be thought of as the tractable problems (Cobham's thesis), so NC can be thought of as the problems that can be efficiently solved on a parallel computer.Arora & Barak (2009) p.118 NC is a subset of P because polylogarithmic parallel computations can be simulated by polynomial-time sequential ones. It is unknown whether NC = P, but most researchers suspect this to be false, meaning that there are probably some tractable problems that are "inherently sequential" and cannot significantly be sped up by using parallelism. Just as the class NP-complete can be thought of as "probably intractable", so the class P-complete, when using NC reductions, can be thought of as "probably not parallelizable" or "probably inherently sequential".
Just as the class P can be thought of as the tractable problems (Cobham's thesis) so NC can be thought of as the problems that can be efficiently solved on a parallel computerArora & Barak (2009) p118 NC is a subset of P because polylogarithmic parallel computations can be simulated by polynomial-time sequential ones It is unknown whether NC = P but most researchers suspect this to be false meaning that there are probably some tractable problems that are "inherently sequential" and cannot significantly be sped up by using parallelism Just as the class NP-complete can be thought of as "probably intractable" so the class P-complete when using NC reductions can be thought of as "probably not parallelizable" or "probably inherently sequential"