Usually an ERP software consist of various modules that are independant but also integrated.

Meaning:

1) You can use each module as a stand alone product.
2) But if you use more than one modules simultaneously, they would communicate with each other by transferring data and by sharing information, and hence resulting in a smarter and efficient information management system.

An Example:

Lets suppose there is a company that manufactures laptops.  This company decides to implements an ERP system.  A team of consultants would come in and study the business requirements and would implement an ERP system.  As a result each department of the company would end up using one more more modules.   Finance department would use financial module. Manufacturing department would use manufacturing related modules. Human Resources deparment would use human resources related modules etc.  The managment of the company could run reports that would display relevant data from accross the board.