I am trying to update my resume after having completed a rotational summer internship this past summer (3 groups within the firm, 3 weeks with each group). Because my time with each group was so short, the program was more about learning about each group's business and seeing where you may fit, and as a result I didn't complete any real work of substance or any one major project. I'm wondering what's best to include in bullet points on a resume: should I choose a project and describe it? or write that I gained an understanding about some industries and markets? or write a few words about each of many small projects I worked on?
Why not just be straight forward. It was a rotational summer internship. Some professions expect these types of internships to be done while still in college. They show that you had an interest in doing more than making Pizza. You went out and found a position related to your future career.
Some internships are tied to a specific project or goal. You are helping a specific group complete a specific task, or to perform a specific experiment. In others the intern is randomly assigned to a team. Or rotated through several teams.
I all cases you showed initiative, and interest, and gained experience and exposure. None of those bad things.
In your resume explain where the internship was, what type it was, and what you learned or accomplished.
I agree with @mhoran_psprep, but for different reasons in addition to the reasons stated.
It's worth listing the highlight of the breadth of skills and business components you were run through. In business there is a certain value to a narrow and well established skill set, there is also a certain value you in a much broader skill set that may not be as focused. Depending on the company's needs the broader skill set may be far more valuable.
Traditionally speaking you hire narrow focused people for very specific jobs or temp work that you need very quick turn around on. You prefer people with a broader skill set to deal with overseeing processes. Basically, if I need a DBA I will prefer someone with technical skill over broad skill. If I'm hiring a team lead, I want a reasonably blend. If I'm hiring a manager or project lead I want the broader skill set.
IE what you got may actually be MORE valuable than a more focused program. (Especially if this is your first entry level job. The vast majority of entry level people have no clue how business works in the real world)