General Objects
Tree
That’s only a design element with no function.
Fire
When you are big and you have eaten a fireflower, you can throw fireballs to your enemies (not all enemies will be dead if you throw it to them).
Box
When you jump on its bottom, some item is jumping out.
That can be (surprise, surprise!)
Brick
A simple box where you can’t go through with different colors.
Pipe
A simple pipe with no special function.
Level-End-Objects
When you touch some level-end-object, you have finished the current level.
House
Congratulations, you ‘ve finished the current level.
Go and jump/run into it.
Igloo
Same as above.