并行分布式计算的同步问题

看论文时候看到同步／异步词汇，不理解。查了资料，主要D.P. Bertsekas and J.N. Tsitsiklis. Parallel and Distributed Computation. Prentice Hall, Englewood Cliffs, NJ, 1989.这本书
书中88页阐述
In any parallel or distributed algorithms, it is necessary to coordinate to some extent the activities of the different processors. This coordination is often implemented by dividing the algorithm in “phases”. During each phases, every processor must execute a number of computations that depend on the results of the computations of other …
我想扫描进来，手打太麻烦。