What is the difference between loaded and unloaded patch panels? Is it simply that loaded patch panels come with endpoint sockets and unloaded patch panels do not?
As you've surmised, a "loaded" patch panel is loaded: Connectors are pre-installed (usually all one family of connectors: e.g. the whole panel is going to be RJ45 cat6), while an unloaded patch panel is - well - not. Typically unloaded panels have empty holes (or knockouts) where connectors would go.
There are a few practical differences as a result: connectors in loaded panels are often permanently mounted, so if a port gets damaged it's dead forever. Unloaded panels let you swap out damaged/defective ports at will.
Some unloaded panels are also designed to allow you to mount differing connector types in the same panel (e.g. RJ45 and Fiber) in whatever proportions make sense for you. This can be an important consideration if you will be scaling up an environment...
Many telecom guys will also argue that it's easier to do cable management and scale-up with unloaded panels (where you would need to pre-wire EVERYTHING with loaded panels it's much easier to add in new ports/runs with an unloaded panel, particularly ones that allow you to install new ports from the front).