The difference is one of focus, macro vs. micro. A big-picture look at the overall story vs. zooming in to sentence structure and word choice. Most of the developmental edits will come in the form of an editorial letter, while line edits are made in the manuscript file, either in comments or directly to the text itself.
Typical developmental edit suggestions might include tips for handling backstory more elegantly, beginning the story later, fixing plot holes, heightening the suspense, raising the stakes, or fleshing out a character’s motivation.
Line edits smooth out the rough edges of the prose. They involve identifying specific things such as grammatical errors, misused homophones, repetition, lack of continuity, point-of-view slips, abrupt transitions, out-of-character lines of dialogue, verb tense inconsistencies, filter words, writing tics, and unclear or awkward phrasing.