Examples of this include (but are not limited to):
* Stores using another companies cleaning services for cleaning its floors etc (rather then having employees of their own hired as full time cleaners).
* An ISP using a third party for its e-mail services rather then using servers of their own.
* A company in need of call centre services using a third person company rather then building a call centre of its own.
Companies often outsource to save money. However, it's also possible they want to concentrate on their main service and allow another company to worry about secondary ones (ie, companies whose main service are those secondary ones).
Outsourcing unfortunatly costs jobs domestically when companies outsource overseas. This is why people dislike it so much.
See outsource and globalization for more definitions.
Oh, and the executives are pocketing the profits too.