How do I concatenate text from detail records into one field?

I seem to recall an example of concatenating or “rolling up” into a single field the values in a text field from records of a child object. For instance, I’d like to show the names of relevant (i.e., filtered) students in a single field on a class object. I could create a table, but it would be smaller and more simple to see in this case with just a quick list of their names. I don’t know how to do it, though, so if I’m remembering right, can you point me to the relevant example?