What is the difference with "放着“和”放到“？

桌上放着一个苹果。It's a state.
把苹果放到桌上。It's a action.

放着：放置；搁置。
放到：表示把某个东西从一个位置移放在另一个位置。表示位置的变化。例如：c从a放到b。有位置的变化。

放着: to place; to put aside.
放到: means to move something from one location to another. Indicates a change in position. For example: c is put from a to b. There is a change of location.

"放到……“is more like an action. to put something on