Get the use of these words wrong, and an editor likely will have to follow up with the admonition to better proofread your writing next time.

Follow-up with a hyphen can be used in two ways:
• An adjective meaning “following an earlier event”: The reporter made a follow-up call to double check his facts.
• A noun meaning “an event that follows an earlier one”: After breaking the news story, the reporter wrote a follow-up for the next issue.

Follow up with no hyphen has a third meaning. It’s a verb showing what one is doing: After covering the fatal accident, the reporter will follow up with a story about the deceased’s life.

“Followup” with no space or hyphen sometimes is used as a replacement for “follow-up.” Editors and instructors generally eschew this spelling, however.