Transaction objects support Event Driven Programming. That is, you can include code inside a certain Event and it remains idle until the Event is activated.
For example, an Event can be associated with a control and when the control is pressed the Event is activated and, if it has code, it is executed.
Furthermore, there are Events triggered automatically, too.
Some of the predefined events available in Transactions (to which you can add code) are:
IsValid Event for Web Applications
Rules & Events Editor
Click event
DblClick Event