There are literally dozens of people on this site more qualified to answer than me, but I'll talk about why I tend to pull putts and what I've done to work on it. I am only going to look at mechanics of stroke/grip/alignment and not going to go into type of putters and what not.
1. I have a bad habit of opening up my shoulders to the line. So, with a putter square to my target line, I used to hit a lot putts with a glancing blow that drifted right. When I'd see this happen, I'd compensate by making sure that putter is closing through the stroke. The result? Putts that don't wobble but that do pull dead left.
2. My other bad habit is the breaking down of my wrists through impact. Specifically, it's my right hand (bottom hand) that releases and closes the face. I just really try to focus on quieting my hands and completing the stroke with the big muscles (shoulders, back, core). It actually feels like my right side (shoulder, ribs, lats) get pulled through when I make a good stroke that holds its line.
3. I've also noticed I lose the connection between chest and arms and putt with my arms too much...the idea of the putter being and extension of your chest and having the head move in relation to your chest is one that I really try to feel when I practice putting.
Hope any of this helps. Good luck!