Photos that aren't mine are in my iPhoto app.

Recently I went on a vacation with other iPhone users. While uploading my photos from the trip to my mac (via my digital camera) I noticed that I had someone else's albums in my events section of iPhoto. This person was on the trip with me and said her photos weren't syncing with her iPad. Why are they on my computer now? If i delete them, will they delete from her device? Can she see my photos as well? I have no clue what I am doing when it comes to this kind of problem. I also would like to prevent it from happening in the future.