If you asked me a few minutes ago I could not have told you the difference between a Partial and Full Boil when talking about home brewing beer. Though it seems blindingly obvious now, as a neophyte and only extract brewer I really had no idea as I have only ever done a partial boil.
With a partial boil you will only boil your wort and a small amount of water, usually between 1-3 gallons. Once the boil finishes you simply top off the rest of the batch to bring it to the 5 gallon (or whatever the final batch size is supposed to be) mark and pitch. This can be accomplished using rather basic equipment, including a small brew pot, stove top oven and an ice bath. This is most common method for creating extract brews.
With a full boil you will be boiling the entire 5 gallon batch at once. This requires you to over compensate the total batch size by about 10-15% to adjust for the volume lost during the boil. There are dozens of calculators avaialble online that do this. For this you require some more advanced brewing equipment, including a massive brew pot, a propane brewing burner to get the volume to boil and a wort chiller to cool down that amount of liquid quickly. These are used almost only for All-Grain brews exclusively.
It would seem that doing a partial boil is the simplest, easiest and most cost effective way to brew a beer from extract. So why would an extract-only brewer even consider doing a full boil? Quality and flavour seems to be the general consensus. Enough people have sworn it improves the overall flavour and removes the ‘twang’ extract brewing leaves in most beers. Personally I have never noticed this before, though I image if I compared the two I would be able to pick it out.
I still do not consider myself an experienced enough brewer to attempt all-grain recipes yet. Add in the fact that it also requires another $200-300 worth of equipment, the ability to brew outdoors and the additional time and effort required, I don’t see it happening for a while.