Well lenses are things you put in front of the sensor and they help you take photos. Some give you a wide field of view, others are tele lenses that make far-away-objects appear close by. Some are bright, others not so much. Most have a focus mechanism to regulate the distance of optimal sharpness is. People seem to love their lenses on the Q, so you probably want to get a package with a lens. You can also get adapters and adapt K mount or I think even C and other mounts to the Q.
More about lenses here:
Pentax Q-Mount Lenses - Pentax Lens Reviews & Lens Database
Appears that the Pentax-Q 01 Standard Prime 8.5mm, with its F1.9, is the fastest. Runner up is probably the Pentax-Q 06 Telephoto Zoom 15-45mm at constant F2.8. There are many threads on the forum in praise of specific lenses, you can even find
sample photos. People seem to especially enjoy the wide angle zoom. Some really love the fisheye! And some enjoy using primes.
Of course, some members complain that there aren't
enough lenses for the Q. Then again, some complain there aren't enough lenses for the K mount cameras, either. Its never enough, you see. But I don't think any of the Q lenses is a real stinker, they just each have their own specifics that you might want or not. I would just be careful with the "Toy lenses" and 03 Fisheye and the 07 Mount Shield lens, because those are the most.. extraordinary in their way, and its easy to complain about their output/usefulness.
The crop factor of the Q7 is 4.7. For the previous models its 5.6. According to Wikipedia that I just checked 5 seconds ago.
Keep in mind that the Q is a tiny, fun camera. It is certainly not meant to compete with the K-mount DSLRs, it is not really meant for professional studio photographers. But it does produce photos and depending on the photographer and scene, they can be pretty amazing
And the Q does offer many features that small cameras usually don't - like interchangeable lenses, recording raw image data, focus peaking, etc.