Choice of Distance
A hobby jogger goes with long distances, has no interest at all in even trying shorter distances.
A runner focus on the shortest distance at which he can reach a satisfactory time. He moves up in distance if or when he finds out that he sucks at shorter ones.
Road Races
A hobby jogger goes to whatever are the most popular ones in town. He checks online to make sure there are enough aid stations. He goes with a huge group of people and it's a whole-day thing including socialization in the beer garden, going out for a "recovery meal" with hobby jogger friends, etc.
A runner looks for the cheapest USTAF-certified course. He looks up the guys who finished at the top last year and stalks them on Athlinks.com, Athletic.net, etc. He rolls out of bed, parks 5 miles away from the start line, checks out a few hot a**ses, battles it out in the front pack with a bunch of post-prime, 35-yr-old, former NCAA runners, grabs some free snacks and jets home.
Body Type
A hobby jogger wants to look like the running models on the cover of Runner's World.
A runner doesn't care what he looks like as long as he's achieving the times he wants. Most likely, he is pale and skinny-fat.
Workouts
A hobby jogger does nothing besides easy runs and long runs. The highlight of his week is having done his longest run yet.
A runner does actual speedwork and tempo runs, in addition to easy runs and long runs. The highlight of his week is his splits for his track workout.