To answer the question simply, crochet is done with one hook while knitting is done on at least two needles.The more complicated answer is that they look completely different and sometimes one functions better than the other depending on what project and outcome you're looking for. I'll start by explaining what crochet is and what I feel it functions best for.
Single crochet (sc) is much tighter, which means there are less gaps. It also means that it's much more sturdy and a bit heavy. This stitch is used most often for stuffed animals or other types of garments that you don't want gaps in.
Half Double Crochet (hdc) is a slightly taller stitch, which means it isn't as dense of a stitch. You might use hdc when making a hat or a scarf so that it still holds in body heat without being too rigid like sc.
Double Crochet (dc) is even taller than the hdc, so it's much more airy. This stitch is often used to make blankets. It may have more gaps between each stitch than the sc, but it creates a much lighter garment. A sc blanket would be very dense, whereas a dc blanket would have much more flow to it. You can even make a hat using dc, it just may not be quite as warm, which is definitely something to keep in mind if it's the middle of winter and you want something to keep your head and ears toasty.
That's just a brief overview of what crochet is. I'd be more than happy to go more in depth if anyone has any questions. Just leave me a comment and I'll write another blog post to answer your question.
I personally prefer the look of knitted garments such as sweaters over the look of crocheted garments. I feel the stitch is much more visually pleasing. The knit stitch also stretches more than crochet does. For that reason, I prefer to use crochet to make stuffed animals. A crocheted stitch is less likely to stretch and allow the stuffing to show through. Knitting also allows you to make beautiful cables, so if you like how cables look, knitting is the best method to achieve the look you're going for.
That's the very simple basics between knit and crochet. There is so much more to each art that I may go into at a later time. As I said above, if you have any questions, please ask! Thank you for taking the time to read this to get a vague idea of what the difference is between knitting and crochet. The next time you see someone knitting or crocheting in public, you can ask them what they're working properly and not risk getting stabbed by a knitting needle or crochet hook :)