If you want to do "big" experiments you should make an image of your OS before doing this.
Even when you don't experiment, making an image at regular basis is crucial!
I do this with
Clonezilla.
Boot with Clonezilla as a live CD and make an image of your partion(s) to an other medium.
To backup individual files/directories (e.g. your pictures) you can use
rsync.