personally read into voodoo to see if it's right for you [as raven said, voodoo is the religion hoodoo is the practice] there is nothing wrong with voodoo, as there's nothing wrong with most forms of magic. where voodoo looses it for me is the use of animal bones and other types of things that i morally don't wanna deal with. [scratch that, bones are fine, animal sacrifices aren't, but that's kinda rare]
what your friend was referring to with the jam jar is a honey jar spell, basically you place a bunch of similar items in a jar to attract things [love, luck, wealth] they're highly effective every times i've made one. there is nothing evil about it unless your intent is such. if you don't believe in karma, then that's fine. personally i believed in it when i first started out but i've seen so many good things happen to bad people and vise versa i find it hard to justify myself. however, just because you don't believe in karma doesn't mean bad things won't happen if you curse someone, or meddle like raven said. the best intentions could end badly.
to build on the above example. let's say your friend is rather quiet around you and you feel somethings bothering her, so you ask her and she says she doesn't want to talk about it. you feel things would be better in the open, because bottled up they will just fester. so you cast a truth spell upon her, and that's when you find out she slept with your boyfriend [as an extreme example] now she's said it, but you loose trust in her, and depending on how good you are with forgiving that sort of thing, break up with your boyfriend and probably stop talking to her as well. truth spells is another one that can fall under 'grey magic'. while magic has no colour, it can be categorized by people a lot.
white = good
black = bad
grey = both
grey is when good intentions imposes ones will on another. this is why magic needs to be thought about long and hard before doing anything. typically magic comes down to the casters intent, but in situations where the intent is good, but the casting involves doing so upon another unwillingly, it leaves wiggle room for it to backfire. this isn't necessarily karma, but just negative results. like adding too much soda to vinegar, a little will make it foam up, a lot will make it overflow and spill everywhere.