FreeBSD isn't Linux. It's a complete UNIX based OS. (And pedigree wise, more of a “true” UNIX.)

It's not Linux, meaning that not everything that runs on Linux will run on FreeBSD. There is a Linux compatibility layer you can enable, but it's not perfect. Because of this, some things released binary only for Linux can be a bit of an adventure. (Adobe Flash) And Linux drivers are a no go.

Package management had been more of a real chore in FreeBSD compared to some Linux systems, but the new package management system brings it up to speed with the best Linux has to offer in my opinion.

FreeBSD tends to be pretty lean and mean when running just FreeBSD stuff, your overhead goes up slightly if we're running Linux software.

Most of the software you're familiar with runs on FreeBSD. Feel free to browse around the ports tree. http://www.freshports.org/

The core OS is not a rolling release. The ports tree can if you wish it to be.

Again on the Core OS thing, typically Linux distros are the Linux kernel and a collection of a GNU userland. These discrete parts are all maintained separately, and patched and put together into a coherent OS by your distro. FreeBSD has a “complete” OS maintained together. This complete OS is more bare bones than you might expect, but it means a very stable and consistent back end.