The 18 day schedule is a fixed schedule where you bid preferences each month for tour lengths, days off, etc. The company builds you 18 work days each month around those preferences.

Newhires and pilots in training are on the 18 day training schedule. Basically it means the company owns you outside of required days off, those days being guaranteed off days after training events. After indoc you're entitled to five days off. After aircraft initial you're guaranteed 1 day off for every two days on the road. If you're in a 24 day long initial, you are guaranteed 12 consecutive days off upon completion.

EDIT: I brought up in a previous post that I didn't believe newhires were guaranteed the 1:2 days off after aircraft initial, and was corrected that they were. After looking at the contract, section 19.4(A) on page 146, I believe my initial thought to be correct. This section states initial newhires are guaranteed 5 days off after indoc and 5 days off after aircraft initial training. Someone with access to the contract can look it up for another set of eyes on it, but I'm pretty sure I remember heading out the door again to do 3 bounces around a week or so after aircraft initial.