I'm not an expert, but this stuff is on the periphery of my work, so I try to understand most of what's happening in the space. LXD is sort of a user interface

I'm not an expert, but this stuff is on the periphery of my work, so I try to understand most of what's happening in the space. LXD is sort of a user interface and network API overlay for LXC. Basically, LXD is built on top of LXC to add higher level tools for managing containers. Compared to Docker, LXC has a history of being used for full-system containers (e.g., a "virtual server" that you could log into as a user and do stuff with it like run a full web hosting system on with databases and other crap), while Docker has a history of being service-oriented (one service per container and you don't log into it as a user, except maybe for troubleshooting). Both use the same kernel features and either could be used for the other use case...but history didn't play out that way. LXD, as I understand it, bridges that gap somewhat and people are using it for service-oriented containers too, but retains its use case of providing a full-system container. It also integrates with OpenStack in ways that I don't think were plausible with LXC. It's written in Go (as most current container tools are), whereas LXC was written in C (as most Linux systems-level development was at the time). There's a bunch of container management tools out there and a bunch of container APIs. This is one of them. They all do things a little differently and have different common use cases, but they all make use of namespaces and cgroups and other kernel features to provide a "contained" environment for stuff to happen in, by some definition of "contained".