I began brewing like most homebrewers: using extract only. I didn't start all grain (AG) brewing for several years mainly because of the discussions on the forums which suggested it was significantly more difficult than extract brewing and that it required "expertise" developed over a period of time as an extract brewer. Also, i delayed because of the discussions regarding the increase in cost associated with the additional equipment necessary for AG brewing. I just didn't want to buy a larger (and significantly more expensive) pot. The additioinal cost can be quite high or you can do it on a budget...like here. The expertise required is, well, to be honest, just a bunch of crap. AG brewing is not rocket science. Don't get me wrong, doing extract first will make the brew process a tad bit easier, but not enough to justify doing extract first. But, if one chooses to do extract first, after you've done one extract session you have all the experise you need to jump to all grain. My guarantee: If after doing your first AG session, you feel like you're over your head and need to run back to extract for more expertise, I'll buy you a beer. ;-)
So, how do they differ? Well, with AG you are creating your own wort, in a process known as mashing and sparging, rather than pouring wort-extract out of a plastic jug. Here's one procedure for doing so.
Step 1: Collect your water for the mash. Normally, I try to keep my water/grain ratio near 1.5 qts/lb. I also aim for about a 3 gallon batch rather than a full 5 gallons, so I reduce my 5 gallon recipe's to 3 gallons (multiply all ingredients by 0.60). So a 10lb recipe becomes 6lbs. At 1.5 qts/lb, I’ll need 9 qts for the mash where (1.5qts/lb * 6lbs = 9 qts). Step 2: Heat the mash water to strike temperature. I use the infusion calculator at this site, http://www.brewcalcs.com/ to determine the correct temperature for the strike. You input grain temp, the amount of grain, and water/grain ratio, and the target mash temperature. So, for a recipe with 6lbs of grain at room temp (75), 1.5 qts/lb, and 152 mash, you’d enter: 75, 6, 1.5, and 152; hit calculate and it will return strike water temp = 162.3 and 9qts. So heat your strike water to approximately 162. I usually round up to nearest 5F so I’d heat to 165. This will allow for a little cooling of the water while I stir in the grain. You can use this calculator for any size recipe.