magic is energy found everywhere in nature. spells use this energy to effect the casters life in some way. this energy is neutral, it is not good or bad, 'black' or 'white'. the intent the caster has when preforming the spell will determine if it's positive [say a luck spell] or negative. [a curse] there are some grey area's, but i'll leave that alone.
since magic is energy in nature, it has limitations, it obeys the laws of nature. as humans, we cannot transform into other beings, fly, levitate, turn invisible, or travel through time. magic can't do this. however, if you wish to, you can do past life regressions with meditation and visit past lives. it isn't magic, and you're not physically going back in time, but that's the closest you're going to get.
hopefully that cleared some stuff up. try finding a book for beginners to help you grasp magic better. are you of a particular pagan faith, or as just interested in magic? i don't want to recommend a book on Wicca to find out you're practice Norse paths.