Primary and Secondary fermentation

I use all grain for my homebrews. When the instructions suggest 1 to 2 weeks primary then 1 to 2 weeks secondary what is the difference? I guess what i am asking is does it mean you can ferment in primary in one week and then secondary a week then bottle! What is the difference if you ferment just 1 week in primary and 1 week in secondary compared to doing 2 weeks in both? (Irish Stout)

Time heals all wounds...The longer you let it alone for the yeasties to do their magic the better. You may end up with yeast still in suspension after a short 1 week ferment, resulting in cloudy and yeast tasting beer.

You could skip the secondary and leave it in the primary for 4 weeks or longer. Just personal preference at that point.

I've seen those instructions a lot in kits. There is no advantage whatsoever in leaving a beer for a week or 2 in the primary then a week or 2 in the secondary. I'd just leave it in the primary for a month then bottle. I only use a secondary if i'm aging a beer for an extended period. Always make sure that beer is done fermenting (check with a hydrometer) before racking to a secondary.

do breweries usually transfer into a secondary fermentor?

Thanks for the info!

I would say at least 2 weeks in primary. Secondary isn't really an issue. I'm only planning on using a secondary now if I need to dry hop or add some sort of fruit or something to my beer. If you are really trying to rush it, 2 weeks will probably be fine although I'd go for 3 weeks min, 4 weeks preferable. I brewed a Guinness clone a little over a month ago and drank it all this weekend at a party with my friends. In my opinion, after 2 weeks you are just going for clarity. With an Irish Stout you don't really need to worry about that. IMO