A service is a fixed piece of work with a published price and a stated turnaround — a pricing page experiment, a caching layer, a landing page build. You know the cost before you talk to anyone, because the price is on the page.
A project is work that has to be scoped before it can be priced: a whole website, a marketplace, a migration. You describe what you need and sellers quote against that description.
The rule of thumb: if you can name the deliverable in a sentence and it has an obvious end, it is a service. If the answer to "how much?" is "it depends what you want", it is a project.
You do not need an account to ask. Requesting a quote takes an email address. An account is only needed once you want to keep a thread, compare quotes side by side, or pay.
If you are not sure which you need, the marketplace will ask a few questions rather than hand you a list to filter.