What is Plabs?
What most people call "Plabs" is actually the "Plabs Network", a routed internet based around an DMVPN-style encrypted on-demand meshing exchange network. Plabs *itself* is the loose group of nerds that keep that network running and have fun (ab)using it. But don't worry, even we call the Network "Plabs" because we're just as lazy as the next guy.
Okay, but what does that mean in practice?
It's routed. That mostly means that we do not need to rely on NAT or similar. If you are a client in Plabs you get an IPv4 address all to yourself and every other IP in Plabs knows exactly how to get packets to you. This is really nice because we can for example use SIP without terrible hacks and work-arounds or can actually use UDP for *something* useful that's not a strict client-server schema.
It's a (partial) mesh. This way there is no single point of failure. If a hop on the path you were using to get your packets to the destination goes offline the transient routers can instantly start using a different path. The absolute worst-case situation will result in less than 5 seconds of connectivity loss assuming there is at least one path remaining to your destination. It's not actually a mesh. Normally in a mesh you need to explicitly set up connections to each node. Plabs uses a few trusted bootstrap servers (called core-servers) that you register with. These core servers establish trust with each endpoint allowing any node to a) bootstrap a mesh using the list of other registered nodes and b) establish trust to any node using a trust chain of a maximum length of 2.
It's encrypted. All the connection going over the open Internet are encrypted using IPsec with some (hopefully) decent crypto.