For those papers that have not been accepted, some people write "submitted to" in their CV and some others use "to appear in". I want to know the difference of "submitted to" and "to appear in". Thank you.
Two small additions to the other answers:
First, I have rarely seen "submitted to", but more often simply seen "submitted," with no journal or conference listed. The reason is simple: where you've submitted to makes no difference, since anybody can get any sort of trash rejected by a top journal. For a "to appear," however, the venue should always be included, since it means you've passed the standards of peer review for that publication.
Second, there is a third category that I have occasionally seen, "in revision." For areas that have a very slow review process, this can be useful for distinguishing that a publication has passed at least one round of peer review, even though it is not yet "to appear." It's still a pretty weak distinction from "submitted," however, so I wouldn't advise using it except for occasional edge cases with high-visibility journals.
For those papers that have not been accepted, some people write "submitted to" in their CV and some others use "to appear in".
Does anyone really use "to appear in" to refer to papers that have not been accepted yet? In my experience (mathematics in the U.S.), "to appear in" means "accepted, but not yet published", and it would be unethical to use it to describe papers that have not yet been accepted. It might occasionally happen, since not everyone acts ethically, but you wouldn't want to get caught doing that.
I can't rule out the possibility that other fields use these terms differently, but I'd strongly recommend sticking with "submitted to". It describes an objective fact, while "to appear in" is at best a prediction and could be viewed as an intentionally misleading statement.
This is somewhat implicit in the other answer, but to spell it out: "Submitted" means that it has been submitted to the cited venue, but has not been reviewed and accepted yet. "To appear" or "in press" means that it has been accepted and is working its way through the rest of the publication process. If you know the exact edition of the journal or conference proceedings that it will be published in, "to appear" with a precise citation is probably more useful than "in press" since it points the reader to the exact place it will be findable and at what point in the future.