Single Hop Onions are, as the name suggests, only one hop away from their entry to the Tor Network (as a client you will still hop through 3 relays), this speeds up access to the service being hosted at the cost of anonymity for the server hosting the content. Visitors to your .onion will still be fully anonymous and protected from evesdropping by their ISP. As an added bonus your clearnet domain will be protected by SSL too! Multi-hop Onion Services (a.k.a Hidden Services or the 'Darknet') are protected by at least 3 hops through the Tor Network which improves the anonymity of the server and helps prevent its discovery by adversaries. If an adversary can't identify the hosting provider for a server they can't force the provider to hand over information about the owner. For some publishers this can be a life or death situation.