The other posters are right-- they are Day Lilies (Hemerocallis botanical name). They are very messy, high-maintenance plants. The reason that they are called "Day Lilies" is because the bloom only lasts about a day before it starts to shrivel up. SO, technically you could be out there every day snapping off the dead bloom, and yanking the dead leaves from the base of the plant.

They will need to be cut to the ground in the fall and should sprout fresh foliage for you in the early spring.