Every modern database system provides functionality to make sure that the system can survive a crash in case something goes wrong or somebody pulls the plug. This is true for filesystems and database systems alike.
PostgreSQL also provides a means to ensure that a crash cannot harm the data's integrity or the data itself. It is guaranteed that if the power cuts out, the system will always be able to come back up again and do its job.
The means to provide this kind of security is provided by the Write Ahead Log (WAL) or xlog. The idea is to not write into the data file directly, but instead write to the log first. Why is that important? Imagine you are writing some data:
INSERT INTO data ... VALUES ('12345678'); ...
