Does building an app or software system count as computer science research? I understand that this topic is a bit vague, but if someone could give me a pointer in the right direction, I would greatly appreciate it.

I'm currently an American undergraduate taking part in an internship at a Japanese engineering university in a professor's research lab for the summer. I have no prior experience with computer science research, and my understanding of it is pretty fuzzy, but I assumed it would involve a decent amount of theory and less software engineering practice.

However, many of the graduate students in my lab are working on projects that I would only think of as software engineering. For example, one is developing a system to effectively generalize how various sensors could interact with a common server. Another developed an iOS app and a server backend that basically gathers data from the app into the server for analysis (and the analysis isn't anything particularly special or innovative from what I understand).

The professor's main research project is developing a web app that uses a special algorithm to analyze discussions on a forum. The algorithm part is what I would think of as research, but from the papers I've read regarding it, they seem to focus on the app as a whole and less on the algorithm.

Do all of these things count as computer science research? Would they perhaps fall more under computer engineering research with less emphasis on computer science theory? And (at least in America, where I plan to attend graduate school) is this a common kind of research for graduate students and/or professors? It just seems strange to me that what I view as basically building a useful app counts as academic research. It seems more like engineering to me and less like research.
What counts as research is a matter of opinion, and ultimately, up to the community of researchers who do research in that area.  It will depend on the novelty of the work, what we learn from it, and what has previously been done.  Generally, just building software usually isn't considered research if no new generalizable knowledge is created; but it can be considered research if it is aimed at developing new knowledge (e.g., evaluating a new technique for solving some problem we previously didn't know how to solve).

You are right: just building a useful app isn't normally considered research.  However, this can be tricky to tell from the outside, because developing a new technique to solve some problem can be research, and if you have a new idea that you think might solve some problem, often the way you to evaluate that idea is to implement it and see whether it does indeed lead to the claimed improvements -- which from the outside can look like you're "just" building a useful app.

So, telling research apart from engineering can sometimes be a bit tricky, especially for someone new to research.  Also, understand that this is somewhat subjective.  One person's research can be another person's "mere engineering"; the community doesn't always agree on what ought to be publishable.  Ultimately, this comes down to a judgement call, one that has to be informed by deep knowledge of the area and also by a sense of "taste" that is developed through experience.  We're unlikely to be able to answer whether a specific project of yours counts as research, because we lack that deep domain knowledge of the area, and because the description in the question is too shallow to tell.  If you're new to research, you might also have a hard time judging this.

If you're new to research, this is why you work with an advisor who is experienced in research.  Identifying useful problems to work on that are research-oriented and likely to lead to publishable results is one of the primary roles of your advisor/mentor.

Of course, if you don't know whether you can trust your advisor, and you don't know how to make those judgements yourself (because you are new to research and not an expert in that area), then you have a more basic problem.
Therefore, let me suggest a useful proxy.  Is the professor's work getting published in reputable peer-reviewed forums?  If it is, then at least that research community considers it research, which is good enough.  If it isn't, then you have reason to be wary.  Thus, with this idea, you can look at the publication record from that group over the past 3 years or so and see what their track record is.

You might also find it interesting to read a few of their past papers.  Often, published research papers try to identify what their novel research contribution is (either explicitly in the introduction, or implicitly when they discuss related work and compare to it).