I have read some articles, and to tell the truth, I could not clearly see the differences. I would like to know if there are differences or not.
If the nose landing gear (NLG) makes contact first, but the plane settles down on the runway, that's a nose-down landing.
If the landing is normal, i.e. the main landing gear (MLG) touches down first, but the plane bounces, only to settle down on the MLG, that's a bounced landing.
If the plane bounces, after scenarios 1 or 2, but proceeds to land on the NLG, leading to multiple bounces on the NLG, that's a porpoise landing. Risking structural failure.