An early bloomer = somebody who peaks very quickly, like a high school sophomore who runs 4:15 for 1600m but can not go any faster than that, and slowly declines after his sophomore year.
Upside of being an early bloomer is you'll be fantastic and get a ton of recognition in HS but after those 4 years, you arent much.
Late bloomer = someone who isnt that great or spectacular in HS (like a guy who runs 17:00 5k by senior year) but slowly peaks until he maxes out by age 30, when most runners slowly begin to decline.
Upside of a late bloomer is you might be great in college and even better in your 20s, but you dont get the recognition you want in HS.