craftgrrl | Stupid question time

Text only, my apologies. I'm crocheting some stuff that I'll be posting when it's done. I'm working on things to donate for needy families this Christmas.

Which brings me to a question. I like crocheting purses, but they're floppy. I keep reading about interfacing, and well, I'm a dolt. Do I sew it in or do I iron it in? I don't have a sewing machine and stink at hand sewing, so ironing would be great! I'm trying to find a used sewing machine though and then I get to teach myself how to sew. (sigh)

But what is interfacing other than it makes the purse stiffer and not-so floppy? Is it expensive?

I'll post pics later today or tomorrow to make up for the text only. Please help a sewing dunce and tell her about interfacing! :)