Technically I think bbPress is a glorified plugin for WordPress. It will not run on it’s own and requires the WordPress database to function. So still essentially WordPress and that is where the styling and appearance comes from. The home page and other bits apart from the forum are using WordPress in it’s basic form as a website.
John
Not sure I would class it like that. From the way things are built I suspect it is quite different to a plugin.
Depends on what you understand by a “plug-in”. In WordPress a plug-in is an optional lump of software that can be hooked into the core and called when events occur.
Many Plug-ins are black-boxes, others publish their internal code. Free and paid for, simple or complex. They conform to an API so you can write your own. Seems very flexible. A user site can consist of a single plug-in, or many parallel plug-ins. As plug-ins can load other plug-ins, a large website can be a complicated mesh of hundreds of interacting plug-ins, including bespoke code.
Dave