Authorship on scientific articles can be a touchy subject. There are several levels of contribution to a work of science, and they are reflected in authorship. Here's how it goes:
1st author: Did most, if not all, of the experiments. Probably wrote most of the text, as well. Sometimes people can get 1st authorship without doing the experiments if they make a deal with the person who did do the experiments but can't be bothered to write the text. The first author gets the street cred for this work. The paper is referred to by this person's name.
2nd (or other middle) author: Contributed something substantial and necessary, like a whole experiment (referred to as a figure), but not as much as the first author. Middle authors get credit for contributing, but you pretty much know it wasn't their idea or project. Sometimes (like "my" Osheim et al. 2005 paper), middle authors only get to be middle authors because the first and last authors are really really nice and/or don't want to argue about it.
Last author: Paid for it. This is the professor in whose lab the work was done (by the first author). They usually contributed important guidance and instruction as well as cash. They get credit for the idea as well as the paper, and most importantly, get to list the paper on their CV so that they can get more grant money to pay for more projects.
Acknowledgments: Back at the end of the paper, these people are listed among the grant numbers used to pay for the work to show that they did something of value, like supportive work (pouring plates, discussing ideas, experiments that seemed like good ideas at the time but didn't make it into the paper).
So for a graduate student, an acknowledgement is garbage, 2nd author is ok, but first author is really what you're after. I present to you my first first-author paper. (There should be another within a couple of months, but it won't be as good as this one.)