The west of a place, country, or region is the part of it which is in the west.
西部 • Many of the buildings in the west of the city are on fire. 该城西部的许多建筑都着火了。
The west part of a place, country, or region is the part which is toward the west.
西部的 • ...a small island off the west coast of South Korea. …韩国西海岸外的一个小岛。
If you go west, you travel toward the west.
往西边 • We are going west to California. 我们将往西去加利福尼亚。