Business Applications also known as Line of Business or LOB applications for short, are terms used to describe applications used in business.
Typically businesses develop specific applications or highly customise off the shelf applications for their own use.
Examples can include a customer relationship management business application which keeps details of customers and interactions with these customers.
How about a specialised investment bank trading application for their traders to use?
There are many examples of in house developed applications that is, the business have their own development facilities and coders to create the software, or use a third party for developing software according to their requirements or maybe it's an application which has already been developed and is being customised by a software supplier or again using their own development people.
Application complexity can be immense but for user acceptance testing, understanding how commonly used functionality is used, can make acceptance testing these complicated applications much easier.