Is there a difference between "forthcoming" and "in press"?

I think many people would construe "in press" as meaning "has been refereed, accepted, and will definitely appear sometime", while "forthcoming" is often more vague, and may include cases where something has not yet been approved by a referee ... or even submitted... or even written.

I use in press for any accepted papers, regardless of the time-to-publication. I don't feel that indicating an article as in press implies that it will be published any time soon.

I specifically do not use forthcoming because it is ambiguous to me. I've seen people use it for everything ranging from accepted papers to future work in a conceptual phase.

The major separator for manuscripts is whether or not they have been accepted for publication. This means been through peer review and revisions and accepted by the journal editors. Once a manuscript passes this step it can be assigned as in press. Anything before that stage is manuscript in preparation or in prep. for short. In a CV you can of course list manuscripts that have been submitted but not passed acceptance and submitted but this is usually not a formulation accepted as a reference in a journal. In my own CV I list manuscript as either submitted or published (which then includes in press).