What is this on my bathroom ceiling fan? ?

Take a hand vacuum and suck the lint off.

If you have a fairly decent vacuum cleaner you can vacuum it. That what I do with the bathroom fan gunk and the split system air con gunk. Just feel the bathroom fan gunk first to make sure it's not wet - but I bet it won't be, so you'll be fine. You can wash the bathroom fan cover after that if needs be. (I've never used the gunk so many times in a sentence!). Good luck! :) 

You'll want to take the cover off, just loosen that knob in the middle, and wash your cover in soapy water then check the fan blades.  The reason you have a fan in your bathroom is to take the moisture out when you are taking a shower.  The steam from the shower build up in that small room and will cause mold, so be sure to run the fan until all the steam is out of the room.  You get dust in the air and so the moist air mixes with the dust and it all gets sucked into the fan.  

It kind of looks like your lungs.

Dust plus cooking grease (like from deep frying or using lots of oil in frying pan.  Take the cover off and wash in liquid dish soap with a fingernail brush and hot water until clean again.

It's dust and moisture.  The bathroom is moist after a shower and when dust clings to it, that's what you get.  You need to put the fan on for a while after a shower and dust more often.