In the U.S. there are four families of snakes that people need to be aware of because of venom. There are many more types of venomous snakes than this, after all just take a look at all the rattlesnakes around the country, but all native snakes that pack that extra punch in the United States belong to one of these four families.
Copperheads. Copperheads are sometimes thought of as an "Eastern" snake because the far majority of them are found east of the Mississippi River. They can be found in Texas and the lower Midwest, such as Kansas, and while copperheads are not aggressive, they're not timid either and that boldness can lead to encounters with people and accidental bites. They are very recognizable because of a distinct copper and green color.
Coral snakes. There are 3 different species of coral snake in the United States, all of which are found in southern states. They can be found from North Carolina, following coastal states all the way to southern Arizona. Coral snakes are the only native venomous snakes to the United States that aren't pit vipers, and as such normal boots are just as good as snake proof boots for these guys since there aren't any fangs to penetrate. Fun fact: coral snakes are technically related to cobras..
Rattlesnakes. These snakes are the most common type of venomous snake by a wide margin, and the only ones that can be found in each and every one of the 47 states that have venomous snakes. The famous diamondback rattlers can grow as long as 7 feet or even more, while smaller Western cousins like the pygmy rattlesnake only occasionally exceed 20 inches. These are some of the deadliest snakes in all of North America, and need to be respected and left alone in the wild. Most are timid towards people and will run away if given the chance.
Water Moccasins. Also known as "cottonmouths," these snakes are all black with a cotton white mouth and are found exclusively in the south, and in small patches in the Midwest, especially around swamps. These snakes are known for being dangerous and overly aggressive in attacking people. They can often be found in large "colonies" and should be avoided.
Those are the four families of snakes in the States that you really need to look out for if you're going to be spending a lot of time off the beaten path and outdoors.