A yellow jacket has it's nest mainly in the ground (though it will build in tree trunks, stacks of firewood, etc.). They have a stinger, like a wasp or bee, and their tails are stripped black and yellow - thus the name. When they sting, their stingers are so very tiny, you can barely see them but you can definitely feel them until you get them out. Unless you can see it, using masking tape (or any tape) to pat on the area will help get the stinger out. They do release poison but it is not as potent as that of a red wasp - unless you are stung by more than one!!!
As for the water moccasin, it is a very poisonous snake that can live both in the water and on the land. They are black in color and definitely found here in the south! You mostly find them where water is (ponds, lakes, rivers - which is all around where I live!!).
Hope this helps you to somewhat understand what they are
And if I've mistated anything or left information out, please feel free to correct or add to what I've posted.