I will start with your question regarding the "starter", which is a type of yeast, in fact it replaces the commercial yeast that would otherwise be necessary in your dough. You have likely eaten sourdough bread at some point in your life, this is the starter (often referred to as sourdough starter) that you see mentioned around these forums. Some of us have gone to the extent of purchasing the "bacteria" in dried form from a source in Idaho who has gathered these bacteria from around the world and has dried them and packaged them for sale to fellows like us who then get these starters going and I am proud to say, my family has not bought a loaf of commercial bread in over three months, and I am now making almost all of my pizzas (averaging about four 13" pies a week) using nothing but flour, water, salt and "starter". And the best thing is, my pizzas are just getting better and better. Oh, and with just a slight bit of care, I expect I will be able to get many, many years of use out of this one batch of starter, which is replenished with each use.
As far as my focaccia recipe goes, I simply use the same recipe I use for pizza but substitute all purpose flour in place of high gluten, and I add a couple tablespoons of light olive oil. As Bill noted, a higher hydration level is needed but I don't measure the flour anymore, I know I will get scorned by some for saying that, but I am now mixing and kneading all my sourdough mixes by hand and so I have found no need to measure. I simply put the flour, salt and water in a bowl (with the active starter of course), mix with a spoon until no longer able to mix, then turn it out onto a solid surface countertop and start adding flour while kneading for several minutes until I get the right feel for the dough. I simply make the focaccia dough a little "stickier". Then allow to sit for several hours or overnight if possible and then prepare as shown in the photos.
I am really glad I invested the time to make this starter culture work, it has added immensely to my joy of baking, and pizza making in particular.