Lazarus offers various events that you can use to enter your own procedures to handle things that happen in your application (e.g. a user clicks a button).
There are rather a lot of possible events to cater for a lot of different scenarios. Somebody who does not know Lazarus or Delphi could well pick the wrong event.
The Lazarus tutorial has some information on what events do what.
Typical event order for forms is:
OnCreate => OnShow => OnActivate => OnPaint => OnResize => OnPaint => ... OnCloseQuery => OnClose => OnDeactivate => OnHide => OnDestroy
This is the equivalent to the class constructor in forms. Use this to initialize form-level variables etc. The form is not yet shown at this time.
When the form is shown (e.g. when loading the form or setting its
.Visible property to true), this event is fired - just before the form is visible. This allows you to modify the visual appearance of controls (e.g. disable certain controls) without flickering.
Fired after the form receives focus. This occurs after Form.Show.
At the OnActivate stage controls instantiated in OnCreate are present and correct, and should have the minimum needed properties set (such as Parent) plus anything else OnCreate specified. OnActivate signals that this form now has focus, so mouse/key events will start arriving.
Fired after the form loses focus.
The equivalent to a class destructor in forms. Use this to clean up/free variables.
When the main form of an application is destroyed, the application terminates.
These events apply to various controls. Please use the Object Inspector to check if it is available for the control you're currently using.
The OnEditingDone event for controls can act like the Validate event in other programming languages: it indicates the user is done changing the control and intends to keep that value. The program can now check the control content for correctness, show error messages, update database fields, etc.
Applicable Delphi information
Because the implementation of events in Lazarus and Delphi is similar, a lot of Delphi-related documentation is applicable to Lazarus with minor modifications.
Delphi documentation is often handy. You can search for the actual control you want to know more about to get a list of its events.
If you want to know about the order in which events fire on a form, this article from about.com can be useful.