See jubeat notes and jubeat ripples notes for a basic information. This page only discusses a changes from ripples to knit. As always jubeat@wiki is very helpful for those who can read Japanese.
The most significant change in jubeat knit is the matching system: it now allows different patterns (of same song) to be played in the same room. Due to this change the user interface was also changed to select the song first and the difficulty then. A player will receive a bonus point if he/she chose a difficult pattern and defeated players that chose an easier pattern.
There is no significance of grade points to unlockable songs now, instead the WOOL system (a metaphor for knitting) is introduced. Players receive a WOOL point according to the achievements, and use WOOLs to knit several “shirts”; they give a new song when the particular shirt is finished.
There are 184 songs, which includes every songs in ripples (110 songs) and APPEND (additional 21 songs). As a comparison one song out of 59 songs in the original jubeat is unavailable in ripples. Due to this enormous volume Total Best Score measure in ripples was abandoned (if it were retained the maximum TBS would be 552m), and the measure for recent performance, jubility, is introduced instead: it is a number from 0.00 to 10.00, with the higher jubility meaning stronger player, and as it measures the recent performance it can decrease sometimes. The exactly formula for jubility is still unknown, but experiments indicate that if you can always get SSS rating or better in level K pattern repeatedly then the jubility will top at K.00. Therefore getting the jubility 10.00 is trivial: just repeatedly play the easiest songs in level 10 (typically Russian Snowy Dance or Kick it out). If you prefer.
The most disappointing thing in knit is the billing of score data: the only free data available seems to be jubility, and due to this change jubegraph gave up the automatic update. It looks like JubeatInfo does the automatic update via a known loophole of the current members site though.