are the great northern bean and the Green bean the same plant just one is harvested by the whole pod in snaps or was i misinformed?
Common beans, which are those of the genus Phaseolus, are used at three different stages of growth, snaps, shellies and dry beans. Green bean is just another term for snap bean.
Any common bean can be used at any of the three growth stages. However, some varieties are considered better suited for a specific use. Great Northerns, for instance, are best known as a dry bean. But you could try them as snaps or shellies.
It's time we had a glossary on this forum, isn't it? There is lots of terminology related to beans. Maybe we can work on a glossary in the off season.
Some people call "snap beans" by an older name - "string beans". This is because at one time, most green beans had to have the strings removed before cooking. This is still the case with many varieties, including some favorites for flavor as green beans. Some varieties of common beans also develop fibrous pods when quite young. On the other hand, some beans which are great as green beans are less tasty than other varieties when used as "shelly" or dry beans. I don't know how Great Northern rates for quality as a green bean, but it is renowned as a dry bean and is probably also good as a shelly.
If you're new to growing beans, it's best to try a variety noted for quality in the stage you want to eat it. Or grow one which is noted for quality at all stages. "Black Valentine" is one of these. Some people love Pinto beans as green beans even though the string must be removed. They are also good as shellies or dry beans.
You can also choose whether you want to grow "common beans" as pole or bush beans and choose varieties which do well in your climate. For example, "Contender" bush bean does well in the South in spring, but develops fibrous pods if grown in cool fall weather (though still valuable as a shelly or dried bean). "Provider" and "Bountiful" do not do well in dry, hot weather, but are old favorites in New England for snap beans.
Lima beans are a different species and are used only as shellies or as dried beans - the pods are not eaten. Many cowpeas (still another species) can be used as snaps, shellies or dried. But usually, "snap beans" refers to common beans.