I only see one photo attached to your question, but that appears that it might be a buckthorn plantain. In any case, the University of Minnesota Extension Service defines a weed as: "any plant that is considered undesirable, unattractive, or troublesome, especially one growing where it is not wanted", and by that definition this is a weed and should be removed.  Although I am providing information to links that help identify plants commonly considered weeds ( http://www.garden.org/weedlibrary/index.php?q=show&id=2938, http://www.pestid.msu.edu/weeds-and-plant-identification/ and http://www.extension.umn.edu/garden/diagnose/weed/) , for purposes of your annual flower garden, anything that YOU did not intentionally plant or sow in your garden area can be considered a weed and can and should be removed.  The reason for removing weeds is to prevent them from competing with your desired and planned flowers for nutrients in the soil, moisture, and even sunlight (if they are shading out the desirable plants).  A way to help you become familiar with how the leaves and stems of your desired plants look is to sow each variety in it's own little space; in this way it is easier to visually identify the outliers, without having to necessarily spend the time to identify each particular type of weed. This is a good article on planning and planting an annual garden from seed.  Planting your annual garden with plants that you purchased from a nursery is even easier, since anything that appears that YOU did not plant can be considered a weed and removed. Once your plants have become established for the season (a few inches in height), you can mulch your beds to help keep unwanted weeds at bay as well as ensuring your annual flower beds have adequate moisture retention.