init is used for when the object is created, ie one time only, and as the first thing. The equivalent is destroy, which you rarely use these ARC days. but if you ex called malloc in init, you would have to call free in destroy.
onEnter and onExit is called each time the node gets active / inactive. Ex when you change scene, the app is pushed to the back, etc. This is normally low overhead stuff like enabling touches, and not something like loading textures.
didLoadFromCCB is not a native cocos2d function. It is called when the component has been loaded with SB, ie after init is called, and after the component has been set up. You could use this to programmatically load some stuff / setup after the SB component has been loaded.