A friend is someone who calls you up, tells you about a sale at an arts and crafts store that is close by but not easy to get to without a car, and offers to swing by and pick you up.
A knitter is someone who notices that your plan to knit your first sweater may be slightly complicated by the fact that you bought chunky yarn instead of worsted weight. The yardages are very different. You can probably get most of a sweater out of the chunky. You will have to swatch and do maths (there is no way around it, you do not want to wear a bright blue potato sack), and buy more yarn at some later date when you know how much you need. D’oh!
A friend who knits is someone who is back at the arts and crafts store the next week, notices one more skein of the yarn you bought and picks it up for you.
Big thanks to the Knitting Librarian! (Who really needs a blog.)