Cross promotion is where two or more businesses agree to promote each others different products. This is a good way to build your business and to help other business owners build theirs.
But I like to use the term "relationship building" when your business success is helped along by others you tend to do business with. For example, if you get to know your banker and he provides good service for you, you will tell your friends and business associates about him and possibly create new business for the banker.
That also works in reverse; if you build a relationship with your banker and he knows and understands your business AND feels a connection with you he will probably be one of the best promoters of your enterprise in town. This is also true of your accountant, insurance agent, etc.
Build relationship on a personal level with others in the business world and those relationships can be a vital part of your businesses success and prosperity.