Der Erfolg is a noun. Erfolgreich is an adjective. That's the difference.
So if you want to say "I have success" you say:
Ich habe Erfolg.
If you want to say "I'm succesful" you say:
Ich bin erfolgreich. (Literally "I am rich in success")
See? You made the noun into an adjective by adding -reich to it.
There are many endings that you can add to a noun to form an adjective. You can find some more examples in the links bellow: