(This is for Kris The Noob, in the comments. She seems to think it was a stupid question, but it seemed pretty intelligent to me.)
Specifically, she'd heard of people taking things off knitting needles to get an idea of size, and wondered how to do that. I'm also gonna throw in a comment or three on how effective it is.
In the end, all you do is pull the knitting off whatever needle it is on, smooth it out on a flat surface, and see what it looks like. It's a way to get an idea if shaping is working, stranded color is laying flat, that kind of thing. If you are using a 'sticky' yarn like wool or mohair, that stays where you put it and takes effort to pull out (the Icelandic lopi I'm using for the Steeked Jacket is a fine example), by golly you can just pull it right off and then put it back on again.
If, however, you are working with the other 95% of yarns that will unravel if you look at them funny, you should thread the stitches onto a smooth piece of yarn or thread with a needle (a sewing/darning one) as you take them off the knitting needle. Same idea as when you put stitches on a holder, and use a spare bit of yarn to do it. Make sure that there's enough extra length on your waste yarn so that when you spread out your knitting, the stitches don't fall off the other end. The waste yarn will keep everything from unravelling, and you can put the stitches back on the knitting needle that way.
I hope this makes sense. I'm drugged again.
A few things to keep in mind when doing this:
A lot of yarns change character dramatically when washed, so you may not be getting an accurate idea of size, even after taking it off the needles and laying it out flat. (If you don't believe me, check out my gauge swatch example, here.) Wool yarns, cable patterns, and ribbings are particularly bad for HUGE shifts in gauge, but nearly everything changes a little bit after washing. (If you're thinking, you can measure your gauge swatch before washing and after, figure the percentage of growth, and then use those figures to estimate what's going on when you knit your project. That works.)
Stranded color will often be lumpier before washing, and then smooth out after a decent blocking. So unless your stranded knitting is HORRENDOUSLY puckered, taking it off the needles won't tell you much. And if it's horrendously puckered, you can tell while it's ON the needles.
It's only worth the bother if your project is mashed up on the needle to begin with, like if you're knitting a sweater forty inches around on a twenty-four inch circular needle. If you can spread out the width of the fabric you're creating across a couple double-points or a really long straight needle, there's no point in taking it off the needle unless you want to try it on. And even then you can often fake it - I've tried on socks when they were on double-points before, and sleeves.
So, taking knitting off the needles is useful. Except for when it's not.
How's that for helpful?