Plantain (Plantago) is one of the most common edible and medicinal weeds around. It is definitely in your yard (unless it has been chemically treated to kill weeds.) It's along roadsides, growing through the cement, and anywhere you find grass. There are two common kinds around here: Plantago major, with broad round leaves and a long green flower stalk (flowering now), and Plantago minor, with thinner leaves and a long stalk which has a smaller top, encircled by little white flowers (like a UFO).
Pictures aren't totally clear...you may do better with an internet image search, or Wikipedia. The blue flower in the background is not plantain...it's a bachelor button that found it's way into the picture. (Also edible.)
The leaves are edible and can be used in salads, though now that the plant is flowering the strings in the leaves get tougher and they are less tender and enjoyable for salads. I marinated this bunch that I picked the other day:
...and put them in the dehydrator for delicious chips. (Marinade = olive oil, lemon juice, tamari, garlic and chili powder.) I have also been adding them to smoothies. Notice the parallel veins of the leaves - that is one way to identify them.
Medicinally they can be chewed up and then placed on bee stings to take the pain away. You can also steep the leaves in olive oil for 2-6 weeks. The oil is very soothing to the skin and relieves itches; great to put on insect bites. In Chinese medicine the seeds are used to treat urinary and bladder infections, and also for constipation.
The book Edible Wild Plants by Elias and Dykeman states that there are no poisonous look-alikes. Still, it is best to have someone show you the plant before you begin to use it if you are unsure about identification. Plants are powerful medicine! Please visit our services page if you'd like me to come and show you and your family and friends what is growing near you.
(By the way, this is different from and unrelated to the plantain that is like a banana. That genus is Musa, while the genus of the green I am talking about is Plantago.)