I really can't make any sense of Heroku's offer. If for example, I create a Rails app, and I want to upload it to Heroku, do I need to pay something? I would like to provide the client something he could play with, but I don't know if I need web dynos or worker dynos just for this.
Heroku has a free offer. So if you don't pay, you have one dyno and can use it.
Very simple to test without big traffic.
The difference between dyno and worker is a dyno is use to web rendering and worker to background job.