According to Mr. Jake Parker.com, Inktober is:

Thirty-one days, thirty-one ink drawings.
 Are you ready?!
INKtober rules:
 1) Make a drawing in ink (you can do a pencil under-drawing if you want).
 2) Post it on tumblr (or Instagram, twitter, facebook, flickr, Pinterest or just pin it on your wall.)
 3) Hashtag it with #inktober
 4) Repeat (you can do it daily, like me, or go the half-marathon route and post every other day, or just do the 5K and post once a week. What ever you decide, just be consistent with it. INKtober is about growing and improving and forming positive habits, so the more you’re consistent the better.)
 That’s it!

Here are my drawings for the first five days, which I’ve been posting, and will continue to post, on my Facebook account and Twitter feed: