The Web Part Life Cycle

Method/Event	Description
OnInit	Handles initialization of the control.
OnLoad	Handles the Load event.
CreateChildControls	Creates any child controls that are used as part of a composite control.
EnsureChildControls	Ensures that CreateChildControls has executed. Use this to ensure that a control you are referencing exists before accessing its data.
OnPreRender	Handles or initiates tasks such as data loading that must complete before the control can render. Asynchronous page tasks should be started from this method.
Page.PreRenderComplete	The page fires the PreRenderComplete event after all controls have completed their OnPreRender methods and the page has completed asynchronous tasks.
Render	Renders the entire control, including the outer tags and Web Part chrome.
RenderContents	Renders the contents of the control only, inside of the outer tags and style properties.

Rerefence: Inside Microsoft Windows SharePoint Services 3.0 by Ted Pattisonand, Daniel Larson