The two are often interchangeable, but they can carry a wide range of meaning.
On the informal side, this "question" might actually be more of a greeting than a question. For example, I might pass a co-worker in the hallway, and say, "How're you doing?" and expect not much more than a nod in return, or maybe a quick "Fine, how are you?" In the latter case, we might be walking past each other as my colleague is speaking, so there's no expectation of an answer from me.
That said, on the other extreme, these questions can also be used to express deep empathy. Another co-worker might tell me that he's lost his mother two days ago, and that he's having to make funeral arrangements. In that case, I might ask him, "How are you doing?" and my mannerisms would be completely different. The words would be spoken more slowly and compassionately; I'd probably be making eye contact, or I might even have a sympathetic hand on his shoulder. I'm essentially asking him, "Is everything okay?" I might even follow it up with something like, "If you need anything from me, just let me know."
In the middle, imagine a group of friends who get together weekly for some activity. In that case, "How are you doing?" might mean, "Do you want to share anything that's going on in your life?" If I have nothing to say, I might respond with a quick, "Good, and you?" but if I have something I want to share, I might launch right in: "Good! We took Elizabeth back to college last weekend. How was your Christmas?"
Lastly – and maybe most importantly – regional/cultural differences may apply, a fact that was brilliantly noted in this funny commercial. By the way, in that YouTube video, I noticed that someone left a comment:
I had to explain this commercial to my friends that aren't from the Northeast.
So, maybe a little explanation is in order here:
- All but one of the characters are from the New York City area. (You can tell from the accents.)
- The man in the cowboy hat is probably from Texas, or somewhere near there. You can tell from both the accent and the hat.
- In the U.S. Northeast, "How ya' doin'?" is simply a quick verbal greeting. It's essentially the equivalent of "Hello, nice to see you." It's not a question.
- In the U.S. South, "How are you?" is a question often used to express interest and invite discussion.
So, notice the facial expressions the first two times the Texan starts talking. Everyone is confused, wondering, "Why does he think we've just asked him a question?"
Their facial expressions are even more priceless after the last patron walks in. At 0:24, the bartender tries using a quick hand gesture, as if to say, "Please don't ask that guy next to you how he's doing!" but it's all in vain; the mannerism is just too ingrained. A mere four seconds later, those looks are hilarious, too, as the new customer is bewildered, and the bartender starts wondering how many times he'll have to hear about the big airport in NYC.
I've lived in both areas of the U.S., and even though I've probably watched this commercial a hundred times, I still laugh heartily every time I see it. It's a wonderful dissection in the nuances of English usage. (I don't think it aired for very long, and I wonder if that's because this "disconnect" still largely exists, so the ad confused more people than it amused.)
Incidentally, nobody is being rude – they are simply unaccustomed to how the expressions are used outside of their respective locales.
As a footnote, I think the Texan believes "the people sure are nice" because everyone is taking the time to ask him how he's doing!
In many contexts, the meanings are pretty much the same, but you might hear finished more often than completed in casual conversation. For example:
I've finished my shopping.
She finished the song.
He finished the race.
I could use completed in those sentences – the meaning wouldn't change, but the register might sound off.
The word completed can convey some sense of accomplishment. In the context of a race, it might work when the race is a major achievement:
He completed his first marathon last year.
Homework, though, is not really a major achievement, so I think you'd hear finished more often in casual conversation:
“Joey, where are you going? Did you finish your homework?”
That said, you might see completed in more formal contexts, such as a paper on education, or a course syllabus:
Students must complete six homework assignments during the semester.
Best Answer
“How are you doing?” and “How do you do?” are questions asking about how someone is feeling, although “How do you do?” has a more formal register to it, and I wouldn’t expect to hear it very much in the US, except perhaps in a formal introduction.
As often as not, these two questions are used as a form of greeting, and the person asking the question isn’t necessarily looking for an answer to the question. However, that’s partly contextual. If I’m recovering from surgery, and a nurse asks, “How are you doing?” I’ll assume the nurse wants to know if I’m in pain or not. If I’m walking down the hallway and a coworker asks, “How ya doin’?” as he’s passing by, I’ll assume it’s just a friendly greeting, and answer with something like, “Good, and you?”
On the other hand, “What do you do?” is a question that’s usually meant to ask what you do for a living. In other words, the person could ask, “What kind of work are you in?”
I wouldn’t expect to hear both questions asked without an answer exchanged in between them. So, a dialogue might go something like this:
That dialogue shows what your questions mean, but it’s more formal than most friendly introductions might be. In a more informal setting, that conversation would sound more like this: