All grain brewing is the same process that commercial breweries use when making their beer. They start with the raw ingredients, and end up with beer in the end. In comparison to extract brewing, all grain brewing takes longer because you are making the beer from scratch—a bit like making a cake from flour, butter, sugar, etc. instead of buying a cake mix. In all grain brewing, we extract sugars from grain to produce wort. In extract brewing, the sugars have already been extracted and provided to you in a concentrated form: either liquid or dry malt extract.
All grain brewing can take several hours longer than extract brewing. This extra time is needed to heat water and convert grain starches into sugars in a process called "mashing." Once mashing is complete, the process of making beer is the same as in extract brewing. Brewers use the all grain method because it allows for greater control over a recipe. You get to choose all the grains, all the hops, and the yeast. With extract brewing, you are somewhat limited by the company that makes the extract. You can choose how light, or dark, the extract is, but you do not have control over the types and amount of grains that are used in the extract-making process.
For those that are looking to be as close as possible to the professionals, all grain brewing is the way to go. In all grain brewing, you have the most control over your beer, allowing you to experiment and fine tune your own recipes.
Is all grain brewing for everyone? No. Many home brewers elect to stick with extract brewing due to the time and logistics of all gain brewing. However, those that venture into all grain can find the process extremely rewarding and the beer unmatched. The process itself is not as complicated as it appears. After a brew or two, all grain brewing will become like second nature to you. For most, the extra time and effort is worth it.