I found a snake in my yard today - black with yellow stripes. I don’t know the first thing about snakes, but I know “yellow means caution”. Is this thing dangerous? It lunged at my dog, so I want to be careful. Thanks in advance to anyone who knows a garter snake from a cobra.
Garter snake - not dangerous.
Not dangerous, that is, unless the op is a frog or a worm!
Where do you live? It can help you figure out whether a snake is venomous. In the USA, only snakes with vertical eye slits are venomous.
If you live in the very far southern USA, that rule of thumb needs to be tweaked.
With one notable, and fortunately easy to spot, exception: the coral snake. It has round pupils.
Red and yellow kill a fellow is the way to distinguish between coral and king snakes, refering to the proximity of those colored bands. Just because a snake is, or includes, yellow coloration doesn’t necessarily mean it’s dangerous. Case in point, a corn snake.
To continue what Philster said, in the US the only venomous snakes are rattlers, water mocassins, copperheads and coral snakes. The links there will show variations of each.
ETA: … and Q.E.D.
Yeah, but who wants to get close enough to check their pupil shape?
Pullet
Likes snakes, but knows not to stick my face next to them.
It seems to be a harmless garter snake, there are plenty of those around my neck of the woods.
However I wouldn’t go too close, 1 - Because it is a snake and I hate snakes, 2 - It is a wild animal and therefore should be left alone and 3 - That’s a quite nasty piece of poison ivy the snake is curled up around, probably do more damage than the snake itself if allergic.
Thank you guys so much for the info, and the clarification on the whole “yellow means caution” thing. Now I can walk the dog without fear!
The pit vipers are also very distinctively shaped otherwise. You wouldn’t, probably, confuse a diamondback rattler with a harmless little garter snake.
Are you serious about the poison ivy? Because there are 5 people and 2 dogs who traverse that area frequently, and none of us have come down with the itchy. Then again, I’m turning out to be not quite the nature scout I thought I was, so you may be right. Can I just pull it out and throw it away, or are we screwed since my husband mowed over the area?
That’s not poison Ivy, this is.
Hence my question about where the OP lives, as this snake is very far south in the U.S.
One thing I learned about garter snakes as a boy: if you pick them up, they have a variety of effective defense mechanisms:
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they shit a noxious excretion all over your hands, and it smells truly awful.
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also, they will bite given the opportunity (easy to avoid that though, simply grab 'em by the neck not the tail). The big ones can hurt.
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as a last resort, they will also puke all over you.
Basically, they do their best to make you regret picking them up. Not that I blame them! 
They are not however particularly venomous, though it is apparently untrue that they are not at all venomous, at least according to this:
This was one of those threads that called for a “…Need answer fast!” in the title, wasn’t it? 
If you can take a picture of something in your back yard, post it on the Internet, and ask a bunch of strangers if it’s dangerous, then I doubt you’re in danger.
As noted above, there are few venomous snakes in the U.S. The “dangerous” ones are pit vipers, with very triangular heads and the slitted eyes mentioned above. (The coral snake is related to the cobra, but coral snakes are very calm and often won’t bite if you pick them up. There’s a kind of king snake that looks like a coral snake; they’re both beautiful.) This garter snake in your yard may “lunge” at your dog, but think about it: What danger is there to your dog from a thing that has no arms or legs? Even if it bites you, it’ll be nothing more than a minor irritation. Whatever toxins a garter snake may have are less annoying than the smell it’ll make if you pick it up.
Which isn’t really useful, given that king snakes instead have yellow bands next to red bands. In one of them, the bands go yellow-red-black-yellow-red-black-etc. from head to tail, and in the other, they go yellow-black-red-yellow-black-red-etc.
That said: Almost all of the venomous snakes in North America are pit vipers, and can be easily recognized by their heads. The head of a pit viper is wider and somewhat flatter than the rest of its body, as seen here (I don’t think that one is native to North America, but it’s a good picture of the head shape). The coral snake, found in the southwest, is the only non-pit viper venomous snake in North America, and it has bands going around its body, not stripes along the length of its body. It’s very difficult to tell the difference between a coral snake and a king snake, but really, who cares? Just leave either of them alone. A non-venomous snake is only as dangerous as any other animal its size, and probably less so, since snakes can’t carry rabies.
Your snake has a neck that smoothly tapers into its head, so it’s not a pit viper, and it doesn’t have bands around its body, so it’s not a coral snake, and so whatever it is, it’s not venomous, and it’s not nearly big enough to be dangerous without venom.
Also note that even the venomous snakes we have in North America aren’t particularly dangerous. 95% of rattlesnake bite victims survive even with no treatment at all, and of course a rattler will warn you before biting. Copperheads and cottonmouths are a little more dangerous, but they’ll still prefer to mind their own business, if you mind yours, and they’re small and stealthy enough that they’ll probably be able to.
Red and yellow - kill a fellow (coral snake)
Red and black - friend of Jack. (king snake)
:dubious: Where are these mutant king snakes?
In the gardens by Professor X’s manor.