What a .onion address actually is
An .onion address is the address of a hidden service: a site that exists only inside the Tor network and can only be reached by running Tor yourself.
Hidden services, briefly
A hidden service runs on a Tor relay, not on a server with a public IP you can look up. Your Tor client finds it by talking through the network, and the address is derived from a key pair the operator generates. A v3 onion is 56 base32 characters, lowercase a through z and the digits 2 through 7, and that string encodes the service's public key, a routing key, and a checksum. It is not a domain. DNS never resolves it, there is no registrar and no lookup table, the routing information is in the string itself. The full set of terms is in the glossary if you want the exact definitions.
Why one shop can have several addresses
Each address is its own key pair, and each key pair is its own hidden service. An operator can stand up several of them pointing at the same backend, and from any single address there is nothing that tells you the other two are the same shop. The pages look identical, the account is identical, but the strings are different, and a different string is a different service as far as Tor is concerned. That is why the set of addresses has to come from a signed source instead of from whatever link someone sent you. On AWAZON Market the three addresses on the addresses page are equal, there is no main one.
Why a clearnet proxy is a different animal
A tor2web gateway runs your traffic through an ordinary server on the clearnet. That server sees which address you opened, and if you interact with the page, it sees what you do. For a product page that is an annoyance. For a login form it is a hole, because the gateway can sit between you and the market and read both sides of the conversation. That is why this directory only links to plain .onion addresses and never to a proxied copy. You run Tor, the connection goes through the network, and no middle server gets to watch.
Is Tor anonymous?
It hides the route. Your traffic enters the network at one relay and leaves at another, and neither end sees the whole path. It does not hide what you do on the site. The market sees a Tor connection, exactly like every other visitor, and your habits still leak: the e-mail you register with, the wallet you reuse from another market, the way you write in a dispute. Tor is the plumbing, not the discipline. The longer list of mistakes is in security, and the first steps with a working address are in getting started.